This is the software i have but i want to update to yosemite how do i do that?

I just bought a 2007 imac it is running on 10.5.8 software right now and i want to update it to yosemite but i don't know how? I tried to call tech support but they say my computer dosent come with tech support so i cant figure it out? any help would be greatly appriciated! i don't know if this is a super simple problem or not i'm not a very tech savy teenager?