Listen to your composition and imagine how you'd like it to begin. What would you like the very first thing for your audience to hear?
If you are having trouble imagining something here are two ideas you can try.
One effective way to begin your piece is to have a solo instrument play your main melody. To do this add some blank measures at the beginning of your piece and then paste in your main melody to one of the instruments. Here's how to add measures at the beginning of your piece:
Another way to begin your piece is to repeat the background part to your A Section two or more times before you introduce your main melody. You could even try gradually building up the texture by adding an instrument on each repeat.
Go to your composition and add an Introduction to your piece!