You can speak basic Russian with most people in Tajikistan. Certainly, you can negotiate prices and get directions in Russian. However, younger people and women in some rural areas increasingly speak no Russian at all. Men in their twenties and thirties in the south have probably spent time working in Russia, and are more likely to speak Russian. Older Soviet-educated people in the south are the most likely to speak Russian well.
Russian phrasebooks and learning tools are easy to find, either online or in Dushanbe in various bookstores and other locations.
As for Tajik, you can find several cheap phrasebooks (Tajik-English and Tajik-English-Russian) in Dushanbe, costing only from 10 to 20 Somoni. These are good enough for a tourist’s basic needs. If you are more serious about learning Tajik, check out the resources listed here.
You will not often come across people in the south who speak English.
The most important thing is to learn the Russian and Tajik alphabets (Tajik adds a few extra letters to the basic Cyrillic alphabet). If you can’t read place names and signs in Russian and Tajik, you will be hopeless.