OK, here's what'cha gotta do:
1. Sight down along the fenders when the hood is opened to see if anything is hitting under there when you close it. It helps to have someone open and close the hood slowly while you look inside to make sure there's nothing in there that hits the hood's underside when closed (it's a tight space). Sometimes it's helpful to look at the underside of the hood to see if there are any tell-tale scratches under there from something hitting.
2. If nothing is hitting, then open the hood and look at the latch pin - it's right near the hood lip, and has an end that looks like a skinny acorn with a screwdriver slot through it. At the base of the pin, where it attaches to the plate on your hood, there are usually two locking nuts. Lightly loosen the bottom one (bottom, as you look at it), then use a screwdriver to un-tighten (counterclockwise) the latch pin. Try going out with it about one turn, then, while holding it from turning with your screwdriver, re-tighten the lock nut you loosened. It's important to hold the pin from turning while tightening the nut.
3. Try closing your hood. Don't slam it or just let it drop - you're not driving an American car, are you? Just push it down so the safety latch catches, then push on either side of the hood centerline with your palms until the pin latches (use your shoulder weight). If it still doesn't latch, try turning the latch pin out 1/2 turn at a time until it firmly latches.
4. Once you get it to latch right, look along the hood seam on either side to make sure the hood edge and the fender edges more-or-less line up. You may have to play with the latch pin length a little more to get it to work right AND line up the hood edges, but it can usually be done.
That's it - you're done. Go hava beeah!
Come to think of it, before you go adjusting the latch pin, just try closing the hood as I described above. Catch it with the safety catch, then lean onto the bottom of the hood and give it a good push, right from the shoulders. See if all the seams line up, then, if it seems closed, take the car for a spin to see if it opens. It's not good to let the hood just drop or to slam it, but you might not be applying enough or proper pressure to compress the hood weatherseal enough for the latch to catch.
gn