If you can find the plant in flower, then that will be the easiest way to identify it. Some field guides are organized by flower color, others by plant structure or plant family. Flower color is easiest for beginners. I recommend getting a couple of field guides to start. The Peterson Field Guide series is very good. Newcomb's Wildflower Guide is also excellent, and because it's organized by the structure of the plants, it will help you learn patterns. For example if a plant has paired leaves, square stems, and aromatic leaves, it's probably a member of the mint family.
If you find a plant that does not yet have flowers, make a note of it and return later in the year to see if it's in flower later. That's especially important if you are interested in learning edible wild plants. Many are only edible when not in bloom.
Start by trying to identify plants in a nearby wild area, or even the weeds in your garden if you let them go to flower. Look for a park or nature center in your area that offers wildflower hikes. The following web site has a calendar of botanic field trips that may not be too far from you:
http://sites.google.com/site/botanicfieldtrips/homeDon't be afraid to tell the hike leader that you're a beginner. They will be more likely to take pity on you and use common names as well as scientific names.