A lot of highschoolers usually hang out downtown, San Luis Obispo has a "nice" downtown, and a lot of people like it. It has a lot of small shops (and some larger ones, like Gap and Express) and places to eat and such.
Other than that, there aren't very many hang out spots - there isn't a large mall or anything. But San Luis Obispo is also a really pretty area to go visit nature areas. Like the beach, and there are also many places to hike, there are many public parks in the area and nature reserves. I'd suggest joining a club or a sports team at school to help you meet people and make friends, I wish I had done more of that when I was in high school. If you don't feel like you're really a sports person, I believe the cross country or track team takes pretty much anyone who wants to join, and the team usually attracts fun kids.
Definitely try out this restaurant called "Thai Classic" downtown, if you like Thai food, and High Street Deli if you like good sandwiches. Big Sky is also another good restaurant located downtown to try.
If you know much about the Sacramento education system, the SLO one is similar. All California have certain standards they need to meet, so most of them have a similar curriculum. And there is a high school exit exam, which I think is similar to the "GED" - I believe if you get your "GED" you do not need to go to high school anymore, but you may start taking classes at a college (like the community college "Cuesta" in the area).
Sorry if this doesn't make very much sense, but I hope it helps some how! Good luck with your move.
Also, just so you know, this community, "Cal Poly", is for the university in the area, not the high school, so most of the people in the community probably know more about the university, not the high school.