I suggest reading the Strategy forum and Raging Amish's unit guide.  Also, watch posted replays if you can find any that will work with the current versions of the game.  Ultimately, there's no real substitute for getting experience and for getting your butt kicked.  I think one mistake new players make is to overestimate the importance of capital ships.  Don't build more than two capital ships early in the game (or maybe even at all) and focus on spamming out large amounts of Long Range Frigates.  Also, think military more than civic; if you are on a flank and have an enemy nearby you are going to have to fight, not go eco.