21 year old male and I've never exercised before?

A person in your situation should see a doctor first.

The delivery job has likely helped preserve muscle. I’d choose an aerobic workout.

You could try walking first instead of running. Set a goal for yourself such as walking for 30 minutes a day or walk a mile a day and increase your time/distance gradually.
If you want to start working out at a gym, most of them will have a trainer go through the machines with you on your first couple of visits. I would suggest you use the treadmills or eliptical machines for an aerobic workout. As with all things, moderation at first so you won’t get too sore and decide not to go back.
Also 5’3 and 130 pounds is not overweight.

Isometrics.

Yes, I’d suggest starting a brisk walking routine, maybe try for 2–3 times a week to start, maybe 30–40 minutes at a time. Walking is excellent for a basic, whole body and cardio workout.Combining a moderate weight routine will be beneficial also.
At 21 you are most likely healthy enough, but, rule of thumb when exercising.

Your heart rate should return to a normal range within 5 minutes of a workout of any kind. If it remains elevated you are overdoing it and need to take it down a notch until better conditioning is realized.
Mix up your routine with hills, walk backwards, jog in place for 45 seconds, etc. Mixing up workouts attains the most variety for overall fitness.

Go hiking, biking, climbing, swimming, any extra activities you have available in your area.
Go walking/hiking at local parks, lakes, rivers, etc.
Go for a run with your dog, or borrow a dog. haha

Eating quality protein and cutting back on carbs will also increase your fitness level,but don’t completely drop the carbs. Remember that carbs are for immediate fueling and protein is for brain/body and staying power.
If you stick to a good routine you will see results within the first 3 weeks that will be remarkable.

Get a physical… then buy the Power 90 course (3 disc set, NOT P90X) from BeachBody or Amazon. Start with level 1–2 and only do what you can. Day by day, do more of the program. Do push ups on your knees to start. Move to level 3–4 after 6 weeks. Finish the program in 90 days (thus the 90). Then keep doing level 3–4 forever if you want. It is a good program. If you actually finish the program, and you have caught the bug, consider P90X. You will NEVER regret getting into shape. You are still young, and you can do it. I went through basic training with guys like you say you are, and they did it, and they could not believe themselves-how they felt, how they looked when they got done. You will not believe yourself either. Get on it.

There are several options, it depends on your personality and how you will respond.

Yes walking is a good place to start but it’s not cardio unless it raises you heart beat for a sustained amount of time.

Try infusing fun into a work out, can we say bowling? Improve your game go alone don’t sit down while the pins reset.

You could do a timer get a magazine or some similar material going through a stretching or basic workout routine, one which is compatible to you. Set the timer for 1 min to start and do one of the exercises, then do 10 minutes of your prefered down time activity, 1 min of the next exercise, rinse repeat. You should be able to easily and desire to add time to the clock.

If you want to run interval training might be good, hydrate well, start at a brisk walk for 3 minutes, increase to a jog, remember you control your breathing, keep your limbs as loose as possible, don’t fist your hands, you may struggle at first so alternate between walking and jogging. If you feel like your holding back however in the middle or end of run let loose, ease back into a walk when you feel you have reached the limit. Later you will be able to push yourself harder. Remember to hydrate.

Step up your routine at work. It is now not a workout in that your body has adjusted to that exertion. If you pump it up a notch or two, it should make a difference.

Another thing is speed up chores. Vacuum more often etc. Setting reasonable goals for a week should also help like say work out for 20 minutes 3 times this week. You can exceed it but you want to try to make it.

Hmm what else. Oh consider taking a class. Like martial arts, dancing, swimming, joining a sports team, yoga, etc.

Or going to a gym with a friend. Though if the friend isn’t consistent you might find another reason to go. Maybe a cute so and so that works there. Even if not present, you still want to feel proud when they look at you. Or notice a difference.

Just start by walking. Do your pushups standing against the wall. Get yourself some resistance bands.
If you can afford it, hire a personal trainer at least to get you started on the right direction.
You will feel so much better once you’re stronger.

You are smart to be concerned about your health. If a medical doctor says that you are healthy enough to start a physical exercise program then get some expert training advice. A personal trainer at a gym will be able to take you safely from your current physical fitness level to whatever level you want or are capable of reaching and help you set and reach goals without suffering injuries. And believe me you wand to avoid injuring yourself which is easier to do when you are in a weak physical fitness condition.
Good health!

Is there any particular goal or something specific you are wanting to achieve? For example is it for general fitness, or you wanting to run a half marathon? That’ll also help you decide a good work out plan for yourself.

Also core exercises will help you tremendously, can’t think of a time where the core is not engaged or a big part of an exercise. Push up as you were mentioning, is good example of this. Just holding the front leaning rest or plank position is good way to build up too.

I personally DO NOT recommend weights till you can do proper exercises with your own weight. I find it typically a lot easier to hurt yourself when attempting to use weights or machines when you can barely lift your own body weight. Specially lack of technique, using correct form, proper breathing (if don’t breath no oxygen to the body/muscles) Bad day.

Just my personal opinion.

Try bench pressing light weights, situps and overhead lifts for a bit. Just work up slowly in weight.

Go to runnersworld.com and look for beginners routines. They also have exercise plans as well. It is all free and very good.

Do ANYTHING physical that you enjoy and start SMALL and build UP from where you are. You will be amazed at how quickly your body will respond, and you do NOT need to physically kill yourself to get in shape—all it takes is for you to stretch your physical limits, experience some pain, rest, and repeat. There it is.

Start with push-ups on your knees instead of your toes.

Walk at first, throw in jogging for minute, then go back to walking for 5 minutes, then jog again for a minute, eventually push yourself to jog a little longer at each interval. Start working yourself up to more stamina. Maybe take a class, do you like bicycling? Dance? Weight lifting? Swimming? Just doing anything consistantly will help you build strength and stamina.

Walking is a low impact exercise that almost anybody can do. Simply walk as much as you are able to and stop when you get tired, and over time you will be able to walk longer when you get in better shape.

You have ridden a bike right? Well ride your bike a little more each day. It’s low impact and fun. A great way to see the world. You can turn around whenever you want and it isn’t nearly as taxing as running. Otherwise get some light dumbbells and do as many reps as you can. Increase a few more each day. Nothing crazy. Don’t overdo it or the next time you’re supposed to exercise you’ll just want to even less! If you can find a sport you enjoy playing, go with that. Working out is a lot more fun when you don’t feel like you’re working out. For me it’s hiking, tennis, and biking.