Do you think other musicians will underestimate me?

If you made the band then they will obviously know about you, unless you totally deceive them, and what does it matter if the underestimate you, you’ll only impress them. This sounds more like a bragging thing than a legitimate concern.

Is this a high school band or something more serious?
If a high school band, I’m sure they are just interested in being seen and heard.
If something more serious, you just have to show how serious you are about your music, about practice time, about rehearsels.

Do have you skills in areas other than the melody? Can you write harmony? Can you write a line for different instruments? Do you know something about rhythm? And structure?

A song that is written in 5 minutes might need some more sophisticated fine tuning.

It all depends on how you perform. I think people will give you a chance to perform if you are hiring them or organizing them. Performance is what counts, not what anyone says. You can have all the training in the world and perform badly. You can have no training and perform well. Both are unlikely, but possible.

Try to learn some music theory.

I can create a sing, and tell what each instrument has to play. But I am all about ear. I took music theory and piano lessons a bit, but I am not amazing. Im just concerned of how the pro musicians I want to form a band with react to me not knowing any instrument… On the other hand, I write and sing. But how do they work? How does a band create a song??

@Mandeblind I think you’ll be fine – are you able to convey your ideas to others? And can you pick things up from others quickly? A band will go one of two ways – they’ll either look to one person as a leader to create songs and introduce them to the others (which sounds like what you would be most comfortable with) or alternatively they’ll work together with different people coming up with a idea, which is then taken up and developed by the others. If you are genuinely working with pro musicians just be sure you don’t insult their intelligence by implying yours is the only way.

As for the audience’s perception, it’s all about what the performance sounds like. If the performance sounds like you are an expert and you know what you’re doing, then they think that you are an expert. They won’t know how much/little training you have had. Actually, if you sound goo and have had little training, they might hold you in an even higher regard.

Also, I would highly advise learning music theory. If someone in your band says a word and you don’t know what it means, then it is a bad thing. Some examples include the following:
arpeggio
diminduendo
andante vs allegro vs allegretto
mezzo-forte vs forte vs pianissimo vs sforzando
ritardando vs accelerando

Yes, music theory is very important.
When you say you can form a song in your head in 5 minutes, what sort of songs do you normally form in your head? Also, how exactly do you hear and form them? I’m just curious here :)

Lorei, I form pop/rock songs more in the coldplay style. I can actually tell what each instrument plays, including the drums. I can also create music with my keyboard, using the sound of multiple instruments. All the lyrics come together with the music too. I think its a talent. I have 15 complete songs in mind and I recorded many just acapella but I remember all the music.

Well you have a pretty good musical memory then, and I’m guessing you also have perfect pitch. Have you considered getting classically trained?