There were some Buffy comics before the season 8 comics started, but those aren't cannon. Although the Fray comic may be cannon because Fray did appear once in the season 8 comic. Fray is a slayer from the 23rd century so if you also like scifi you might want to check that one out, it was 8 issues and is collected in a trade. I haven't read it but I've always heard it was good, and was an interesting window into the future. I think the thing with all three seasons of the comic is that you can kind of jump in anywhere, and you'll eventually figure out what's happening especially if you have the ground work of having watched the show.