i am a massive comic book geek. my collection is approximately 4000 single issues and 300 or so graphic novels/collected editions (i've been collecting/reading them seriously for about 15 years, and i even worked at a comic shop for a couple years in high school).
if you're looking to cover a lot of continuity quickly (and cheaply) marvel has published quite a few of their "essential" collections that usually comprise 25 or so issues for $15-20 (dc has a similar line now, but i'm drawing a blank as to their collective title).
comics are a much larger, more varied medium than they generally get credit for, so it's tough to give you advice, not knowing what you're specifically into/looking for. if you're into superhero stuff i would recommend the "essential" collections of x-men and spiderman, and there is a lot of good stuff out there for batman. there is also a lot of really great non-superhero stuff out there, too. one of the best things you can do is to find a comic shop in your area and get to know the owner/employees, they will usually be very knowledgable (which seems like a prerequisite for the job, but it's not 100% true) and can help point you in the right direction based on what you've liked/not liked in the past. also, if you want to post more about what you like/dislike here i could make more recommendations (or feel free to pm me if you'd prefer). like most comic geeks, i rarely get tired of talking about comics.
one note of caution, comics can be a very expensive hobby (i currently spend on average $80-100 a month, and have spent as much as $200 a month), but a lot of libraries are starting to carry some graphic novels and collections.