There is no substitute for playing as much as you can. Whether its on the beach with a doubles partner, or in the gym with a group of adults at the local YMCA, or wherever you can, play volleyball.
There are a lot of great drills and ideas on the internet. You can find a few good drills and then work, work, work. There is no substitute for putting in the time.
If you set the goal of making your high school team next year, work as hard as you can to get better and don't stop until you do. Work on your passing, hitting, blocking, setting (even if you are not trying to become a setter), serving, and court sense. Seek out our high school coach and let them know of your desire to get better and ask them what you need to do.
Also, if you have a local college team, ask the coach if they have a player or two who would be willing to work with you in their off season.