If you've made it past the flop in Omaha 8 or Better, you are probably going to see the hand through until the river unless a disaster card comes on the turn that would prevent you from doing so. If you had a draw on the flop and you make it on the turn, what your opponents do, and how many of them there are, will often dictate what you do. For example, let's say that you have the nut straight on a rainbow board, with no low potential, and there are two others in the pot with you that cannot beat you for a high because they have low hands.
What do you do if a player bets into you with what you believe is the nut low? You might think that raising would be automatic here, but it is not. The reason being is that you do not want to give the third player in the pot a reason to fold. If you call, you entice this player to call and if you're lucky, they might just raise with their low and him and the other player will do all of your betting for you.
This isn't always the action you should take though. If your hand is vulnerable (for example, say you have 2-3-9-10 and the board is 3-6-7-8 with two hearts and two spades) then you should be raising to try and eliminate and punish drawing hands.
If you get to the river and you have a nut high hand that has no chance of being split, it is always correct to raise. Your opponents will not know for sure that you are raising with the nut high. They might think you have the nut low, with a weak high, so they might re-raise with a weaker high thinking that they will be getting that ½ of the pot. Besides if the pot is big enough, people will often make crying calls just because they are hoping to get a piece of the pot. Let them have a reason for crying when they make that call.
As a beginner, it is often best to just play straight forward poker. Bet when you have it, call when you have draws to the nuts or think you will be splitting the pot, and fold when you don't have anything worth proceeding. As you play more, you will start to learn various tricks that you can add to your game to make you more money. Stick with the basics for now though, and you and your bank account will be glad you did.