Well, if you're heading to San Francisco, that necessarily puts you post-1821, since Spain did not allow American emigration, and probably after 1848 when news of gold strikes in CA coincided with the conclusion of the Mexican-American War and suddenly the country was grew by one-third. You'd want to learn Spanish, obviously, and French as well, since the last vestiges of fur trading were still around then. These would be easy enough to pick up in a place like St. Louis, as a center of commerce and the edge of "civilization," as far as white Americans were concerned (although you'd likely have to apprentice yourself to a French or Spanish merchant or tradesman to do it - but learning a trade like saddlemaking or blacksmithing could also enable you to hire on to a wagon train).

Trickier would be learning all the Native languages you'd need. Depending on the exact time, and exact route you took, languages of interest might include: Lakota, Cheyenne (Northern and Southern), Pawnee, Osage, Comanche, Kiowa, Navajo, Apache, Ute, Shoshone, Paiute, Mohave, and more. There are also dialects within these languages that would require navigating (for example, Navajo and Apache are related, both being Athapaskan tongues, but even within Navajo there are dialectical differences so deep that when the Marines were initiating their Code Talker program in 1942, a Navajo from the western part of the rez dropped out because everyone else was from the eastern part and he was uncomfortable with their dialect). I'm not sure where you'd learn these - mountain men, trappers, traders, and missionaries generally learned indigenous languages while living among the tribes. There weren't exactly a lot of, say, Utes living in Missouri who you could learn from, no linguistic programs at the University of Missouri (founded 1839), and there were no phrase books you could page through. Also, if you can't afford to join a wagon train, there's no way you could afford all the gifts and trade goods you'd need to negotiate peacefully, alone, across nearly 2,000 miles of Native land. For more on this point specifically, check out James Ronda's Lewis and Clark Among the Indians. Other sources informing this post are Hampton Sides' Blood and Thunder, Pekka Hamalainen's Comanche Empire, Chester Nez's Code Talker, and Elliot West's Contested Plains.