That said, you don't have to have read any other issue for this one to make sense.
This issue does a good job of explaining how the team got together in the first place. The story mines stories from the 60s, 70s and 80s to tell the tale, but as I hadn't read any of those before reading this and it all still makes sense.
For those that don't know, the Guardians of the Galaxy takes place in the 31st century of the Marvel Universe, where men from several of the colonized planets of the the solar system unite to defeat an alien invasion, then remain together to defend, well the Galaxy.
The best character, in my opinion, is Vance Astrovik (Major Victory). In this reality he is an astronaut from the 20th century that was supposed to be the first man to travel to another star. (The trip takes 1000 years, but when he gets there it is already colonized by humans as they had discovered faster then light travel during his journey). What I loved when I read this initially (back in 1992) was that Marvel was also publishing a comic called The New Warriors, which starred Vance Astro (AKA marvel Boy and later Justice), an alternate version of this Vance who had mutant powers. The two crossed paths in another issue, that was a very rewarding to me as a reader of both series.
Anyway, this first run of Guardians of the Galaxy was pretty solid for the first 30 issues (there is a new volume running by Dan Abnett and Andy Lanning I haven't read yet).
This one is $1.00.