Even warmongers need diplomacy. You can't take on the entire world at the same time until you have a huge military. That's not possible in the early turns of the game. You must keep at least a few nations relaxed while their neighbors get crushed underfoot.
Use trade routes, cooperation agreements, and mutual wars to help people think that you're a nice nation. It doesn't have to be accurate, and you won't have to keep up any pretenses forever. You just need to buy time as you get stronger and stronger.
Don't be the one to start wars in the early game. Instead, wait until the computer comes to you with a request to join them in an attack on a nation that isn't far from your border. This raises your diplomatic rating with the allied nation, and it keeps you from looking as bad to the community at large. Any other friendly nations with the attacker will have a better rating because of the "friend of my friend is my friend" style of diplomacy.
During those wars, let your buddy breach the enemy borders first if possible. This means that they'll hit the hardest defenses. Once that starts happening, send your military in, and try to scoop up the maximum amount of territory. Be really pushy about getting capital cities this way, as they're often great prizes due to long periods of tile improvements and Growth.
When choosing whom to befriend, try to think about affinities. Nations with the same affinity make better friends because it's easier to stay on their good side. If you haven't gotten far enough into the game to tell your rivals' affinity, see how they've reacted to your stance on aliens, at least. Go with people who agree on your alien activities because they are also more likely to stay on your good side.
