The early game is the hardest part against the harder AIs. Once you have some ship techs to choose from in building your fleet, you can exploit the fact that the AI has absolutely no idea how to properly build a ship - your fleets will be able to crush fleets twice their size.

In the early game, wars are tough due to the way the AI cheats at economics. You can either use exploity tricks like leading enemy fleets around with a single corvette and preventing them from engaging, or you're in for long, difficult wars. If they come too early, especially against advanced AI starts, you might have to give up a few planets to get peace. You need to expand really aggressively early on and try to bribe your way out of wars for as long as possible, unless you can box in an enemy. With early wars, even successful ones, you should expect to wipe your entire fleet at least once or twice; the trick is to make sure you're able to rebuild it quickly. Also, prioritize destroying their marine fleets if you can.

The later in the game you survive, the easier the AI gets. They'll probably always outtech you, but that won't really matter. If you're still having trouble, you can try the no clustered start mod - the extra room to expand gives you more time and space to build up before the AI starts looking to expand via war declarations.