The first thing to consider is, what weapons and defenses the enemy is using. Take a look at the enemy ships and see how they are fitted out. Try to use defenses that counter the enemy's weapons, and weapons that penetrate the enemy's defenses. Just keep in mind: shields block beam weapons, armor blocks mass drivers, and point defense blocks missiles. Using a different kind of defense also has an effect, but it is much smaller than a on-type one.
The second thing to consider is the size of the ship. The bigger the ship, the more hit points and space for equipment it has, but it will also be more expensive and you can't put as much of them in a fleet. Try to keep in mind the task you want the ship to perform. For example, if you want a pure offensive fighter with nothing but weapons on board, then use a tiny or small hull. With a high logistics ability you can put a lot of them in a fleet. While they are pretty fragile, they are also cheap and easy to replace, if you lose some.
The last things to consider are speed and logistics. Keep an eye on the speed of enemy ships. Try to position your ships so you can attack them first. If possible, you also want to be faster than the enemy, but that might not always be necessary. Definitely try to get the first strike, because the first round of combat is often enough defining whether you win the battle or not. You also want to have as many ships in the fleet as possible, so a high logistics ability is important. This is especially true if you are on an equal tech-level as the enemy, and even more so, if you are behind.
I hope, this will help you.
