Well, the cafeterias are theoretically built to hold all the people in a given lunch period, it's not like there's not a seat for you at all. My advice would be, if you wanna eat in the cafeteria, it's not that hard. It is pretty much one of the more socially demanding places in school, though. I'm assuming you know that.
Anyway, what you do is, pretty simple, you look around for a free chair at a table, and then sit down. You should ask before sitting, and just be like "is it all right if I sit here?" Something to that effect. It's possible sometimes to just sit like, anywhere, without even asking or caring at all about the social dynamics, and you probably won't get kicked out, but one time my friend in high school sat at a "cool kids" table and then the entire table moved and left him alone at the table. Yay passive agressiveness. Generally, the "easiest" tables are like, the nerd kid tables, so to speak. Then the "harder" ones are the preppy/jock tables. You might surprise yourself and maybe even fit in at a table you don't think you would. My advice for your first few weeks, try a bunch of different tables and see what you like best, and then you'll settle at one or maybe two tables.
Also, it helps, if you have friends in your lunch period, you could start by sitting at your friend's table. However, your friend's friends might not be people that like you, or people you like, so yeah. Another thing you MAY wanna do, is even if you eat at the cafeteria all the time, like, say, one day of the week or so, still eat in the classroom, so that way there you get a break from socializing and don't "burn out" from it, and if people ask why, just say you need extra help or whatever that day. Also doing this, it makes it so you don't "wear out your welcome" with some people, too, and them you.
Good luck.