I would suggest the Total War games, in particular Rome: Total War if your pc isn't particularly powerful. I consider Rome to be one of the best RTS games out there and it has an entire tutorial campaign that can help you get work your way into the main game. The Total War games are quite different that other RTS games though, in that they separate the RTS battle element from the campaign map where you build all of you troops and manage you city's.
If you want a more standard RTS, then I would recommend games by Relic, such as the Dawn of War series, or the Homeworld series.
If you want a more standard RTS, then I would recommend games by Relic, such as the Dawn of War series, or the Homeworld series.