Downloading iMovie

I want to purchse iMovie but when I try to it tell me that I need Mac OS X version 10.7.4. I am currently on Mac OS X version 10.6.8 and I cannot find any information on updating to 10.7.4 because everytime I go into Software Update it tells me that there are no updates for my computer. Please help as I do not understand the vast majority of the discussions on here, I just want to buy iMovie!