Others have addressed some key tree issues.
Here is a site with some basics - look at the links center left, start with the one on building a tree and by all means follow each of the 5 sub -links. It is all well written and does not 'talk down' to you.http://genealogy.about.com/
The LDS' revised Family Search site has a wiki with some excellent articles on how to do specific areas research and other topics:https://wiki.familysearch.org/en/Main_Page
For US research help, the USGenWeb project State and County pages have useful brief history items and many volunteer-contributed resources, arranged by State:http://www.usgenweb.org/
--also check out the network of sites from the link at right labeled "Project Archives".
Rootsweb's home page has very useful links at upper left, including to another research guide:http://www.rootsweb.ancestry.com/
As you get your teeth into the basics, and start needing more resources such as maps, published research guides (really, books can be a lot handier than websites) and very specific stuff, one of the oldest and richest sites on the web can give you stunning lists of mainly internet resources: www.cyndislist.com
Just for kicks, have a look at a site that shows you how some of the really ridiculous trees got started:http://personal.linkline.com/xymox/fraud/hoax.htm
Really, the most fun is getting to the treasure-houses: archives, specialized libraries, historical societies' manuscript collections, Church repositories and Courthouses -- that have untold riches in the form of land, estate, tax, vital and Court records that can provide a lot of details about ancestors' lives. If you just looked at internet trees you would not have a clue that there may be diaries or journals, business ledgers, plat maps (etc.) about one of your ancestral neighborhoods. This is really where you find out that a ggggf wound up in court because he sold an ox on a Sunday, or that an ancestor's land was redistributed because the daughters' guardian declared that the folks who did the original partition gave the boys the most productive land. Estate inventories might list the requisite equipment for an 18th-century cooking fireplace, what sort of dishes were in the cupboard, or carriage-maker's tools.