Well, if you have a bore sighter that can help, or you might be able to remove the bolt carrier group and look through the barrel and get it pointed at something in the distance and then line your sights up with that as a rough approximation.
Most gun stores should have a bore sighter, though they may charge you some money to use it, or there's probably a fellow shooter in the area.
Beyond that, shoot it and see what gets hit. Start close and fine tune it out to further distances.
There is a method I've seen talked about that sounds like it has some serious potential. Bore sight then aim at the bulls eye and shoot one shot, hold the rifle in the same position and adjust the sights, or have someone else adjust the sights, to line up with the point of impact. Obviously this is easier with a rest or gun vice or something similar. Then aim back at the bulls eye and fire a second shot and use that to adjust, then shoot a handful of times and hopefully your sights will be in the middle of the group. There's really no point in stressing yourself out by adjusting after every single shot, especially not as a new shooter because unless you're some kind of savant you're probably going to need lots of experience before you get really good accuracy with iron sights.
When I first took my AR out I'd shoot five and go downrange to see where they hit and then come back and adjust a little and shoot again and go down and check the target and adjust a little more and then shoot again and by the end of it I used about 200 shots trying to get that damned thing sighted in and I didn't have any tape with me so the target was so ragged I wasn't even sure where the bullets were hitting anymore, but it was fun and that's the point.
