The first step in improving your tennis game is 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. They 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 as well as you can.
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 ball. 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 bad hits. 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 with the ball.
Your stance is also going to be very important. If you are 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 the ball. 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. Being able to anticipate where the ball will go will take practice just like the other parts of your game, but you will find that over time you will develop a greater feel for where each ball will land.