My first suggestion is to make a paper template of your pictures. You can gently tape them to the wall to get an idea of whether you like the grouping if your hanging several pictures or doing a gallery wall. Measure the hanger for each picture vertically and horizontally and mark on the front of the template where the measurements intersect.
Once your ready to hang them (heavy ones need wall anchors), a laser level is your friend. They can be spendy, so you likely can rent one from a store such as Home Depot. Otherwise a regular level will do. If possible, use a long level so it spans across a longer run. Adjust the height of the level so the bottoms of the paper templates line up. Since you have the marked the template where the measurements intersect you can place the nail thru the paper and onto the wall. You will need somebody to assist you if you go with a regular level.
Paige: maybe this will help
First of all assess the wall space you wish to use. I would never do the template of the frame suggested because it is the picture in the frame that should determine its placement not the size of the frame. 1) hanging pictures is a very fluid action and is not something to be overly concerned with, because they can always be re-hung. 2) do not hang pictures too high. The other day I visited a friend with a relative of mine and I pointed out how my friends art work was hung properly (she is a decorator) because nothing was too high. Place your things in relationship to what is beneath them, not the length of the wall. Check out Pinterest for ideas on how to hang things.
Have you heard of Command hanging strips? here in the uk people are using them instead of nails, easy to adjust the position of your pictures, and remove should you decide to remove the picture
If you do use nails please do check if there is an electric cable in the wall (you can get gadgets that help you check this by bleeping if there is an electric cable)