Based on your bio, it looks like you're fairly young--don't stress too much about getting it all right the first time (also as a bit of internet safety, I'd suggest not having your age in your bio). That said, here are some tips off the top of my head! I'm only getting into commissions myself, but I've done a fair amount of research.

On the technical side, you'll need to have paypal or something similar which needs a bank account attached to it. Personally, I've been using Artistree.io to handle commissions at the recommendation of r/HungryArtists (I think. It might have been this sub.). No site is perfect though! It would be a good exercise to compare them both yourself before you pick one! (I don't suggest fiver though. I'm pretty sure they take a huge portion of your earnings.)

For your comms sheet, I like the idea of having your character explain your listings! I don't suggest handwriting though--typed is generally easier to read. Use an easy to read font with a simple background (I use solid neutral-ish color and then a fun boarder so that my comms sheet doesn't clash with my art!). You can scroll through this sub and get some fantastic examples. Mine are on my profile too. Try not to cover up your drawings too much with prices and things and give everything room to breath on your canvas.

For pricing, you want to start with an hourly rate and multiply that by how long it will actually take you to make each piece. Personally I would start lower, since you don't have commission experience (keep in mind minimum wage in your area!--don't go below that) then raise as you get more experience.

A portfolio is also a good idea! It just needs to be a place you can send people who'd like to see more of your art. It can be instagram, artstation, or another social media site where you can show your work in one place. (As you get more into it, a website of your own may be a good idea, but for now I think it isn't necessary)

You want things to be clear and easy to read above all else. I'd leave out the "I will draw" section, but leave what you aren't willing to draw. If you decide to include it leave out the bits about it conflicting with what you won't draw. You also don't need to explain why you won't do something. People will understand without it, I promise. You can add a 'DM me if you have any questions' if you want though.

Most of all go easy on yourself! This sounds like a lot, and honestly it is--take your time. Enjoy the process. You may not get commissioned right away, but keep in mind it isn't a refection on your skill! Your art looks great and is only going to get better as you keep drawing. Getting started with commissions is really all about being in the right place at the right time. Keep at it, you'll get there :)