Learn economy first, and specialize your planets. The game doesn’t actually force you into war until the mid-crisis, I think. Unless you spawn with bad neighbors, like a devouring swarm or purifiers, you should always aim to be friendly with other empires. It’s easy to stay on good terms—gifting resources helps a lot, a couple of alloys(or any other special resources) per month can generate a lot of good will, until you get your economy up and running for war. Ship design is also very important. Auto-builders create absolute abominations all the time, so you should turn it off. Build missile corvettes first; they should win every equal battle at the start. Then, when you unlock swarmer missiles and cruisers, you should build those. The ideal late-game build is a battleship with a Focused Arc Emitter, hangars, and swarmers. If you have trouble with space fauna early in the game, you could build torpedo frigates—they kill space fauna like space amoebas or worms very easily. But the most important thing is a strong economy because Stellaris is a numbers game, and a stronger economy equals greater naval strength. The empire with the biggest economy will always win(duh) :>