I'm taking JimShaw's suggestion and making a little informational post for those who are getting started.

Navigation is one of the most critical skills for any hiker or backpacker. I am not going to try to teach anyone how to navigate; there are plenty of good books on the subject, and classes available in many areas. But I want to emphasize something that I think is not always made clear in navigational instruction - and that is the attitude you ought to adopt. You can't just navigate every hour or so, or every time you come to a trail junction - it needs to be a constant awareness of your surroundings and of what the trail is doing (assuming you are on a trail, which as beginners I would expect you to be)and what the terrain is like. And you need to think well ahead of where you are at the moment. Before the trip, you need to study that topo map, so that it becomes familiar. Before you leave the trailhead, look over the part of the map that covers the first few miles of trail. You look at that and say to yourself: "okay, so we go along this valley floor for a ways, then we'll fade to the right and up the side of the valley, there's some switchbacks there, then we follow that draw up to the saddle, and there's a little peak off to the left of the trail as it goes through the saddle, and from there we ought to be able to see that first lake", (if you can't do that,then you don't know how to read a topo map and you ought not to be out there without someone to help you navigate). Now you're ready to go, because you know what to expect on the trail and if it doesn't look like that then you better wonder why not, and figure out why ther trial isn't going they way it seems to go on the map.
When you don't look ahead, you don't know what to expect and you're lost before you know it.
And you need to look at that map often - like really often. I keep my map in a little pouch on my shoulder strap where I can look at it any time - and I do look at it often. If you put your map in your pack, you might as well leave it at home. Same goes for the compass or GPS if you're using one. You want to make it easy to check. Here's what happens when you don't look at the map for a while: I usually cover 2 or 3 miles in an hour on the trail, maybe 1 or 2 off the trail. If I haven't looked at my map for an hour, I can be 1 to 3 miles away from where I thought I was before I have any idea. By paying constant attention, you'll know sooner that the trail you ought to have taken at the last junction was the one to the left, not the one to the right.
Basically the idea is to keep constantly checking your position in a rough and ready way - Should I be able to see that mountain yet, or that lake? shouldn't there be a junction along about here? Sometimes you'll come to a totally unmarked junction - no sign, no nothing. How can you tell if it's the junction you are looking for or just an old trail that's not shown on your map? If you've been paying attention, keeping an eye on things as you go, you'll know that you haven't crossed the stream that should be before that junction and so this must not be it. It's that sort of little clue that you need to be keeping track of.
Once you get used to it, this becomes second nature, and all those little glances at the map become a habit. And then navigating is just part of how you walk, rather than a separate thing.