A problem that people have when learning how to play tennis is learning how to improve their technique. This can be done by learning what you are doing first and then lots and lots of practice.
If you are not sure of what you are doing then working to follow a few small tips will improve your overall game greatly and allow you to play a much better game. Having a good hitting technique is not the only thing that you need to play a good tennis game.
Your first step is going to be the tried and true tip of practice and practice some more. If you never practice your technique then it is not going to improve no matter how many books you read or how many tutorial videos you watch. Learning how to improve your technique by reading some books and articles can be helpful, but you must take what you have learned and actually put it to use on the tennis court in order to actually reap the benefits that are available to you.
Your next concern should be making sure that you are using a tennis racquet that is appropriate for your skill level as well as physical size. If you are using the wrong racquet then no matter what you do there will be no way for you to hit the ball effectively. If you work very hard to ensure that you are hitting the ball then you are certain to ensure that you are making progress. A good appropriately sized racquet will make it much easier to hit the ball and ensure that you are able to play the best possible.
For actual tips, you want to always hold the racquet as if you were going to hit the ball with your palm for a forehand swing. This will allow you to easily get the grip on the racquet you need to connect squarely with the racquet. If you hit from the side your ball is likely to land in the alley and unless you are playing doubles this is not a god idea at all.
For a good backhand, you need to hold the racquet with either one hand or two depending on how you are most comfortable. If you do hold the racquet with two hands you need to ensure that you are not overlapping your hands as this will cause you to tilt the racquet slightly which will again hinder your technique and likely result in numerous balls that go out into the alley. You need to also ensure that you are not using a racquet that is too heavy; if you are, it will pull down either forward or backwards and make it difficult to get a good solid square connection to the ball.
Your stance is also going to be very important. If you are standing around casually slouching when the ball is coming at you and slowly yawning as you lift the racquet you are not going to get the impact that you need. You are likely to not care about your technique either. However, since you are reading this you are telling yourself that you do care about your technique, which means you care about your stance as well. Ensure that you are properly balancing the weight between your legs to get the best results possible.
The last step is to always ensure that you watch where the ball goes. If you are wrong about where the ball goes and are having to suddenly bolt across the tennis court to barely touch it with your racquet you are not going to have as good of a return as if you had anticipated exactly where the ball was going. The skill of always watching the ball will involve a bit of careful planning and practice both on the court and off in order to be truly successful at improving your technique.