Rather easy game to play, a very hard game to get really good at. There are some pretty good FAQ's about the game over at Gamespot. The best offhand advice I can give you is, HIT THEM ALOT!! If you repeatedly hit the ball carrier as soon as the other team gets it, you can really keep the momentum in your favor. Try and use the slide tackle to intercept passes (works like a champ with a fast charc), and if they get it past you HIT EM!!
As far as scoring goes, pass the ball around if you are in tight coverage. Building up the speed of the shot helps a ton. The easiest way to score without using a special move is by passing it around on the ground until it starts getting the bright white trail, then pass the ball while holding Z and you will pop it up in the air and get it to one of your guys rather close to the net. If you hit pass while it's still in the air you'll do the bicycle kick and have a really good chance of knocking it in if you have it charged up. Other than that, you got special moves to score with.
The special moves are of course unique to each of the charcs. The trick is to learn the ideal distance from the goal to use the move. If you find the sweet spot as far as range goes, you will score with the move at well above a 90% rate (if you dont get hit!!). My favorite sidekicks are:
Hammer Bros: Use the hammer throw at medium distance (try to hit the goalie with the 2nd hammer)
Mole: Almost garunteed score from pretty far out, keep your distance while using the move
Bone Turtle: Excellent charc, now the trick with this guy is using his special kick to stun the goalie. When you use his special move, it'll blast a big lightning charged soccer ball at the goalie stunning him. Once he is stunned, run a player up to kick the deflected ball in. You will only get the special kick in the goal about 15-20 percent of the time, all of your other goals will come from kicking the deflection in. This is the best way to keep the heat on the other team. Stun their goalie a few times in the first match and they will start watching him more than your other players because it drives them nuts.
The sidekick with the best special move for scoring is that chic egg shooting monster (forget her name). If you use her move from Med to Long range you have great odds of making the shot. Seems to happen quite often that you hit a player (other than goalie) with the initial shot, if so, hit the next closest person and grab the ball with her and you usually have just bought yourself enough time to get the move off again.
Only other thing I suppose would be the captains. I perfer Yoshi or Pete to do my stomping. Yoshi is a little more rounded charc and his tounge attack is a huge pain in the @*# in the right hands. You can really put a damper on their offense with it, but his special move the egg kinda sucks until you get used to it. Now my man Pete (the plant) is for when you want to get rough. If you play aggressively (aka 50+ hits a match) Pete is your man, and he still has a great shot percentage with a good special move.
I really don't worry myself too much with the 6-shot strike with my captains. Don't get me wrong, if you leave me the opening I'll give it a stab, but when I'm down clearing your defense out and passing around, I'm looking for a single goal. Most of the time I use the multiple shot is on a breakaway or if I'm down on points. Hopefully that made a little bit of sense, wrote it in a hurry. Check out Gamespot's listing for Strikers, like I said earlier there are some good strat guides and charc guides listed there.