Repeat the letters of the alphabet aloud until you are comfortable pronouncing them. Use an online resource such as "Master Russian" that has audio files of the sound of each letter. Listen and repeat the sound until you have mastered it. Some letters in Russian look the same as letters in the English alphabet but are pronounced differently. Don't assume you know the pronunciation of similar letters; start from scratch.
Make a list of words to practice that have a range of sounds. Start with shorter words. As your pronunciation improves, try some short sentences of three or four words. Phrasebooks are a great resource of useful short sentences.
Find Russian speakers to practice with. Try websites like "My Language Exchange" to find Russian speakers who want to practice other languages. You may find someone willing to help you with your Russian in exchange for help learning English. Simple conversation is the best way to gain confidence in a language. The more you speak, the more your pronunciation will improve.