study a lot of pro tennis after learning the mechanics.. it helps... and no you dont need a trainer ive never had a trainer and i consistently beat people who do... you just have to be able to study mechanics correctly and not do them wrong.
at first you will probably hit the ball and it will go surprisingly far, and you will be like what the hell? But after awhile you will learn to hit it harder but hit through the ball, controlling where it goes.
most importantly, hit the forehand and follow through. most players, if given the opportunity, want to hit with the forehand over the backhand. So if you have time to adjust to the forehand side do so. To get topspin on the forehand (thus making it "drop" in the court) you just have to lower the racket before hitting it and bring it up when you hit the ball. The power comes from your wrist coming toward the ball when it makes contact and going through. Also, you want to move your shoulders when you hit it for extra power.
Backhand is similar, and you can choose to do either a 1 handed or 2 handed backhand. (2hand ftw) just make sure you follow through and you may be surprised how hard you can hit it and keep control of it.
Get those 2 down, and then learn to slice (which isnt hard). Easiest way to learn to slice is to play normally and just turn the racket so the racket hits the ball angled. like this: (ball is o, racket is \) o\ when you bring the racket downward, the ball will hit the face of the racket because the racket is "opened up" toward the ball, as opposed to swinging the racket perpendicular to the ball, like this o| then you would have to have the exact timing down perfectly and you will not be successful. you usually dont have to slice the ball with an extreme amount of spin on it, otherwise it will hang in the air longer and be less accurate.
the best thing to learn is also court position and footwork.... After you serve, pay attention to where your weaknesses on the court are. If you serve and the opponent barely gets the ball and is pushed back, you may consider rushing the net and playing close to the net. On most cases, you want to be slightly behind the back baseline (where you serve behind) and in the middle of the court with your feet set evenly and the racket right in the middle of you. This allows you to quickly go either way and is a perfeclty neutral position. after you do a hit, dont sit and watch it or the opponent can attack your weakspot. move yourself toward the neutral positoin and get as close as you can to it. Even if you dont make it back to the middle and behind the baseline, SET YOUR FEET when you opponent is going to hit the ball, this allows you to go anywhere, so if the opponent hits it back behind you, your momentum wont keep you from turning back around.
pay attention to footwork when you hit forehands and backhands too, it is very important.
yea just pm me for more advice thats good for now