kbedwards: Depends on the situation and your long term goals.
If it's very early in the game and the you seem to be alone with just this AI, then move you troops in the opponents land and sneak attack. If you wipe them out before any other contacts are made, the news of your vile deed will die with them.
If it's mid-game and you've met a couple other AI and have run out of expansion room or need a resource they have then, position your troops out of the opponents territory (ships, workers & scouts included) and contact that AI. Check the existing deals list, if there's just a peace treaty, click on that to ask to renegotiate the treaty (sometimes you can demand a tech from a weaker opponent early in the game this way). When you take the peace treaty off the board, your advisor will ask you if you want to declare war, just click yes and go at it. (note: it's normally best to declare at the start of the turn to get the moves (and sometimes first shot) before the opponent can react.
If it's end-game and this is the last opponent, you can set up a ROP deal with the opponent and position all of you attackers next to targeted cities, then declare war and attack (called ROP Rape). This is a one-time thing per game, so make sure you pick your spot. Many players decline to use this (I'm one of them) because it feels like cheating and a dishonorable way to win.
If you're going for a non-Military win, then ROP is not for you. You'll either need the votes or cash (gpt deals) from the AI in the future.
BTW: Welcome to the forum.