I don't know much about this game when it applies to tournaments. But as a general outlook on the game here you go.
You may use different cards for the high hand and the low hand. To make a low, you must use 5 cards that are 8 or below. Always 2 from your hand and 3 off the board. There are times where no low hand will qualify.
Here is an example of a hand:
You hold
Board reads:
Notice that your high hand is a full house, 5s over 8s. You use the two 5s from your hole cards. For the low, you use the Ace and the 2, making a 5432A low, which is the best low possible. The high pot and low pot are separate, although you can win both at the end of the hand, like you most likely would have in my example.
As far as strategy goes, I always like mid-range suited connectors with A2 or A3 as backup for the low. Two pair is not a very powerful hand since omaha is very draw-heavy. Straights are powerful but make sure you have the nut straight. Avoid hands like A5JJ that might be appealing due to the high pocket pair. Your hand must have great high potential, but also have the low backup.
If you want, tell me what site you are playing on and I'll watch and you can join me in the chat. I'm not good at the game, but I can try to help. You'd need Steve to chime in on this for much better advice.