What you need are plant keys. It may not be easy to ID stuff where there's no starting point (you have absolutely no idea what it is). Many plant keys assume that you at least know the genus you are working with. A number of groups are working on universal random-access keys, but no one has yet completed such a wonderful resource.

There are all kinds of keys available on the internet. Unfortunately, all seem to have some drawbacks. eFloras is one of the better keys, but has the disadvantage of currently focusing more on foreign (especially Asian) species. It's still the one I'd choose if I had to stick with just one set of keys. It takes a little getting used to; it's sometimes a little hard to navigate.

There are also many keys that address certain types of plants. The National Arborday Foundation has a simplistic tree key. It's good for common trees. There are also many many other generic and regional (state, county, etc) tree keys out there.

You can also find many lawn weed keys. Here's one link that list quite a few of them: http://www.garden-counselor-lawn-care.com/identify-lawn-weeds.html

For wildflowers, check for a wildflower key. Again, something specific to your area might be much easier to use by eliminating lots of searching through plants you are not likely to encounter.

The Massachusetts Invasive Plant Advisory Group has a list of many commonly seen (because they are invasives or potentially invasive) plants in your state. Just familiarizing yourself with them could be a source of lots of IDs.

Another resource is the Gardenweb "Name That Plant!" Forum and other forums specific to various types of plants.