verbs - I thought I'd buy a gun

Well, I was watching Gone Girl and there's an scene where you can see this text on Amy Elliott-Dunne's dairy (Rosamund Pike).
I've asked people that I know about it, but the only answer that I got was that it means something more like: I thought I'd (I would) like to buy a gun.
Also I was told that this is pretty common during daily conversation. I think it has a name or at least its tense has a name.
I'm not a English native speaker so I'll really appreciate if someone could advise me, and maybe if I made a mistake (grammatically or orthographically) I'd really appreciate your corrections as well.