When you see an unfamiliar snake, try to estimate its length, which in North America will range from less than a foot to more than 8 feet. Consider whether the animal's head is broader than the "neck" leading to it, and the general girth of its body. Is the snake glossy or dull (which relate to smooth or keeled scales, respectively)? What is the skin pattern: uniformly colored, banded, blotchy, striped? Note the snake's behavior; does it coil up to defend its turf, lie perfectly still, or attempt to flee? Also make note of its location, both specifically in terms of habitat and broadly in terms of geography. All of that information will help you identify the snake using field guides and other references.
There are many fine field guides dedicated to North American snakes, including the Peterson series on reptiles and amphibians (split into Eastern/Central and Western North America). Although smaller books are ideal for keeping in your backpack or glove compartment, larger reference editions, such as Carl and Evelyn Ernst's "Snakes of the United States and Canada" (2003), are useful for perusing at home for more thorough information, and may be more likely to contain taxonomic keys. Check your local library or bookstore to see whether there are any field guides specially dedicated to snakes in your particular state or region.
Taxonomic Keys and the Internet
For serious snake ID, you'll want to seek out a taxonomic key, which is usually a series of paired questions about detailed physical characteristics. The key can help you identify a single species that matches the description of your snake. Such keys are no longer confined to expensive technical manuals; there are Web resources of varying complexity that can be of use. The most reliable are often productions of universities, museums, extension offices, or government agencies. See the "Resources" section for some examples.
Many wildlife, natural resource, and extension agencies provide information about regional snakes. The Wisconsin Department of Natural Resources, for example, offers a richly photographed guide to that state's species, complete with Wisconsin-specific details, such as county range maps, that often are unavailable in broader-focused field guide.
Keep in Mind
While a comparative few North American snakes are venemous, you should err on the side of caution in all cases. Don't heedlessly approach an unfamiliar snake. If possible, make your observations without unduly stressing the animal. For novice snake enthusiasts, it's a good idea to learn the potentially dangerous species---rattlesnakes, copperheads, cottonmouths, and coral snakes---so you can quickly identify them in the field and give them wide berths.