Blitzball is one of those things that starts out very rough (the storyline required match, that you luckily don't have to win, demonstrates this readily to most players) but gets easier and easier the more you do it, assuming you know how to work its XP system.
I don't remember all the fine details behind it, but I do remember that you want to pass the ball as much as possible. Even if it's to the point where you're not scoring and not winning the matches, every pass you do is XP for your fielders. Just hot potato the ball inbetween whoever doesn't have opponents immediately near them, potentially pausing only to lure opponents away from other fielders. Nap skills, if you have them, can help with keeping the opposition off of you. As long as the opponent isn't interfering with your pass, it will usually succeed, unless their PA is low and the pass is of particularly long distance. All the passing you're doing will get you XP (using pass skills if you have them, even if they aren't necessary, also gives more XP), and all the time spent by the opponent not being in control of the ball will mean they don't get as much XP. Gradually you will level past them and you'll gain an edge.
The goalie can't get in on this action unfortunately, so make sure to shuffle the positions around so that they don't lag behind.