How do I get better at Tennis?

Alright I have been playing tennis for like 1 year now. From last Sept. to March. I played once a week with group lessons. From April to June I played once a week with a trainer who tought me how to hit topspin forehand/backhand and topspin serve. Right now that trainer moved away and I really don't have any friends that play tennis and it isn't fun playing against the board. My topspin forehand is alright and my topspin backhand is pretty bad, my serve is good when hit right but inconsistent. I start my freshman year this year and wanted to play tennis for my school and it starts in mid March. How can I get better without my trainer and only having the lessons once a week? I want to do real good but I feel hopeless as I don't have the trainer anymore.