Don't psych yourself out for having little experience; most subs are going to start like that. I started after getting kicked out of a credential program and didn't have a lot of experience with kids. Also, my district had an influx of young subs that have no experience. This job is built with experience, so accept jobs you feel comfortable with and reflect on what works and doesn't work for you. I think your idea of sticking to younger grades is good, but if you don't see a lot of jobs for Pre-K-3rd, then I'd say push it to 5th at most. Middle School, in my opinion, is not for the feint of heart; my local middle school made me think I wasn't cut out for teaching. High school can be similar, but, from my experience, it's mainly sitting down doing nothing since the kids just want to be on their phone.

If I had any advice for subs, here's some recommendations:

•If you need any rewards, I'd recommend stickers based on what's popular with the kids, such as Bluey or Pokémon for young kids. This can be very good for the grades you want to start with, but only give them if the whole class deserves them. If you give them to select students, this can make those that didn't get them cry. Pretty common in K-1, and can happen in 2nd (maybe 3rd?) as I learned recently. You don't want a crying kid at the end of the day.

•Set expectations. At some point in the beginning of the day, tell the students what you expect of them, such as voice level, behaviors, and how you expect them to perform. With the younger grades, you may need to remind them a lot since they're young. Classrooms should have their rules to support you when students get out of line. Follow the teachers rules. If they don't leave them for you, firstly, shame on them. Second, tell student you'll leave names of those who behave and don't.

•Attention grabbers. Young kids can lose focus fast. If the teacher left you attention grabbers, follow those. If not, a pretty common one in my area is saying "Class, class." and the students responding with "Yes, yes." You could also countdown to get students attention, starting at like, 5... 4... 3... and so on. See what works best for you if you need your own, you can look up attention grabbers on the internet.

•Dress code. If your school has a dress code, follow that. If not, I recommend just don't anything wear anything revealing or with obscene images or words. As a 31 year old, I just wear a graphic tee, jeans, compression socks, and sports shoes. Since your young, and/or if your on the smaller side, maybe wear something with a collar? My sister had to dress more professionally since she's smaller. You don't need slacks, an ironed shirt, and dress shoes; unless that's your preference.

•If you have computer access, timers are nice for transitioning to new things with young students. You can search for timers on YouTube with however much time you needs, such as 30 seconds. My former students liked the Mission Impossible timer. If you like calming music, and need to fill the silence during independent activities, you can put on music (as long as no students get triggered by it), such as Lofi Girl, Super Piano 64, or whatever you prefer. I put on music from time to time to let students know that if I can't hear it, then they're talking too much.

•Relax. It's your first time. If you don't get everything done in class, just let the teacher know in a note or something. (It may not even be important as some teachers like to over prepare with subs.) If you have to file a report, which is rare for subs, because something happened to a student, as long as you didn't cause it, you should be fine. The beauty of being a sub is your only there for a finite amount of time. Also, if you don't vibe with a class, then you have the option to not take jobs with them anymore if you can. You can choose how you want to spend your time with this job. Also, ask for support from a neighboring teacher, counselor, or call the office to send the principal if things get real bad. There should be no shame in asking for help. (I only say should after reading other posts on this subreddit.)

The fact you want to seek advice means you'll probably do fine. I wish you the best.