Astronomers compare telescopes primarily by the diameter of their main ("primary") mirrors. This is referred to as the "aperture" of the telescope. You'll want to get a Dob with an aperture of either 6 inches (150 mm), 8 inches (203 mm), or 10 inches (254 mm).If you can afford it, purchase either an 8 or 10 inch version: The larger the diameter of the mirror, the easier it is for you to see dim objects (like galaxies and nebulae) in the night sky. Prices range from roughly $230 USD for 6-inch apertures to $500 USD for 10-inch apertures. The telescope should come with two or three eyepieces: If it doesn't, shop somewhere else.  You'll use the eyepieces to control the magnification of the telescope.  For example, with one eyepiece in the telescope, you can see the entire surface of the Moon.  But with another eyepiece, you can zoom in on a crater.

These tools will help you learn the constellations, and otherwise help you get started in astronomy. Also, consider joining a local astronomy club where you can gain experience using various types of telescopes, and get help learning the night sky.
Also, consider purchasing the following accessories:

Nightwatch: A Practical Guide to Viewing the Universe  -- a really good introductory book by author Terence Dickinson;
A subscription to Astronomy magazine -- a monthly publication with beautiful photos, a guide to the night sky, product reviews, and the latest astronomy news;
Some astronomy software -- such as Name A Star Live's Virtual Planetarium software;

A planisphere --  Use this simple, lightweight, handheld device to identify constellations and major stars in the night sky;
An inexpensive pair of binoculars -- These are good for helping you navigate around the night sky as you search for a galaxy, nebula, or star to observe through your telescope; 

A TELRAD -- Most telescopes come with a very small telescope, called a "finderscope," mounted on top of the main telescope.  As the name implies, a finderscope helps you find the astronomical object you're looking for through your main telescope.  But most astronomers prefer to use a TELRAD -- a battery-powered device that projects a red bullseye in the night sky.  It's much easier to aim a telescope with a TELRAD than with a finderscope.

Company Resource: Monocular Telescope    Promotional Binoculars    Gift & Promotional Binoculars