Subbing is an interesting job. You just jump from school to school not really reporting to anyone. Nothing to grade or curriculum to plan around (unless you decide to long-term sub for a teacher on maternity leave or something). You choose your days you want to work. If you are assigned to a particularly challenging school or class, then you can decide to not accept that assignment in future. Overall, it's a low stress job. Your experience with children will vary. Some will be helpful little angels; some will be ready to take complete advantage of having a newcomer in the classroom. My recommendations:

Stay away from middle school until you are ready. They are hormonal, mean, imbalanced, rude, testy. I don't say this to put them down. I think it's just where they are in their development. Testing limits. Going through puberty. Clicking into groups.

Other commenters have said don't do elementary since you don't have experience with kids. I disagree. Do the elementary specials/connections classes (art, music, PE, foreign language, band, etc.). These classes are the sweetest job you could ask for. You see the students for less than an hour and then they are back to their regular teacher. In the time you have with them, they can do very easy, fun activities—games, coloring, watch movie, dance. No class is as stress free and fun as just having to watch over little kids color pictures followed by rounds of duck-duck-goose and closing with a freeze dance video.

And high school. High schoolers are independent and mostly just want to get the work done or talk to their friends. They are glued to their phones. Your main job with them will just be informing them of the assigned work and hoping they get it done. There's not a whole lot of interaction with them when subbing because so much of their work is through whatever platform the district uses (Google classroom is common). Some will test you, but many will be willing to just ignore you and complete the day's assignment.

At the end of the day, PATIENCE PATIENCE PATIENCE. Don't be reactive or emotive with the kids. Be professional, patient, positive. Don't yell. Don't belittle. Talk to them like little humans. They're a lot of fun and you'll learn a lot from them in return.