Almost all astronomy textbooks explain RA and Dec, but if you aren't interested in the technical details, just use the explanations below.
When is the best time to look for a particular constellation?
For each constellation, I have included where on the Earth it is visible from and what month is beast for viewing it. First of all, some constellations are never visible from certain places on the Earth. For example, people in the Southern Hemisphere can't see the North Star (Polaris) because the Earth is always in the way. If your latitude is between the two numbers listed on the constellation pages, then that constellation will be visible at SOME time during the year. (If you don't know your latitude, you can look it up by city on MIT's geography page).
Technical Note: It is easy to figure out which constellations are visible for you. The Declination is very closely related to your latitude on the Earth. All I have done to figure out the maximum and minimum latitudes is add and subtract 80 degrees from the declination.
Figuring out which month a constellation is visible is a little trickier. The month I have listed on the constellation pages is the month when the constellation is highest in the sky. However, each constellation is visible for a range of months, depending on your latitude and its Declination. In addition, the constellations rise and set like the Sun and the planets so, as explained in the Short Instructions above, the time of night that you are out stargazing also matters.