Can any experienced techie minded person walk me through setting up a dual boot system?

For decades I have been using Windows, and in 99% of the different shades of shit it comes in – and I’m kind of okay with Windows 8, but I really want to try Linux and see if I can get my system to give me the option of running one or t’other.

I have experience in installing an OS (Windows obviously), from scratch, but no experience whatsoever in setting up a dual boot system. Or even in installing Linux.

Right now I have a terrible hard drive of woefully inadequate size (90Gb – don’t laugh), which replaced a 500Gb HD in a bit of a hurry to get it up and running again. However, I do have a 2TB external HDD, and was wondering if it would be possible to have Linux installed on a partition on the ExHD, or do I have to clear out my 90Gb HD to install both on the same hard drive in different partitions…..or what?

I really have no experience in setting up partitions, dual booting setups, none of that stuff, so I really need someone who knows what they’re talking about to get me through this. I don’t mind having to come back and spill any extra information if it’s needed, but I do need to find out how to get this done safely without entirely borking up my only system.

I’m not a complete beginner, I can work my way round services and stuff, just not setting up a computer in this way.
Anyone could help here that would be great.