SC2 was my first RTS ever and I was new to SC2 lore as well. Even though A.I. always goes with the same strategy, it is fun.
If you don't like playing RTS for a moment, take a custom map. You got something like LoL which is fan made but also Left 2 Die, Starjeweled and Aiur Chef which are Blizzard made and gives a fresh breath of air now and then. Plus there are a lot more fanmade custom maps / modes.
If you are unsure to buy it, just download the starter edition of SC2. You can play 1 1v1 map + some other maps and a couple missions ( You can only play as Terran though).
PS: You can "abuse" the Terran part if you join with a friend and he leaves, you can control his units, aka other races. (2v1 on A.I. very easy shouldn't be a problem if you aren't new to RTS).