The serve is probably the hardest shot in tennis to master but, it is the one shot that you have total control over, so it's worth spending time to get it right and make it effective.
First things first, just practise throwing a ball from the baseline, making sure that you turn your shoulder a little bit, down the court so you can get good rotation and power.
Another aspect to work on is a consistent ball toss. To help you with the ball toss, find yourself a netball or a basketball net and throw the ball up and through it. You'll be able to see for yourself just how consistent your ball toss is and work to improve it. Spend a bit of time practising this because if you do want to have a great serve, a consistent ball toss is crucial.