The older you get, the higher skill level you should be. You should always try to play against someone close to your skill level or higher to only make you better.
In high school there are 3 skill levels, the highest which is varsity, the lowest which is freshman and in between those two is junior varsity. High school should be the best 4 years of school, so only play a sport if you're up for a lot of competition because that's what you'll be getting. If you're really good and lucky as a freshman, you may move up to junior varsity. This should always be a dream of yours to play with the upper classman because they'll only make you better and you should feel proud.
As for high school volleyball, you'll be playing by the rules of indoor volleyball. At least, I've never heard of a high school team having an outdoor volleyball team. The rules for indoor volleyball are pretty much the same as outdoor.
However, they do vary slightly.
For example, the court dimensions are different for indoor and outdoor and by age. Also, how far you can go out of bounds to save a ball which varies from court to court because you never know what objects are around the court. Then there is the height of the net which doesn't only depend if it's indoor or outdoor volleyball, but age as well.
And another tip, when out on a volleyball court playing a game you need to be able to communicate with each with volleyball sayings. This is to let each other know who has the ball, wants to be set up, etc. However, if you're on a junior varsity or varsity team you need to be able to communicate very well since the game goes so fast.
Regardless of your level of experience, whether you're a freshman, JV, or varsity player, be sure to have fun on the court!