I'll tell you something not to do.
Don't spend a couple decades playing 6-string acoustic and electric guitar, and then think you're going to start playing bass guitar the same way.
You will, at a minimum, annoy other bass guitar players to no end. 
Actually, you will annoy the guitar player(s) and drummer in your band. Other bass players will just laugh at you.
It's not that experienced guitar players can't play a bass with reasonable skill, most of us can, it's just that guitar players generally
doze off do not have the patience to play bass the way it should be played in most songs.
While bass playing can be fairly simple, it still has to be done well. Unless you are only doing something experimental, it requires that you become quite proficient at counting time. This is an important basic part of all musical education, but especially so for instruments that are relied heavily upon to track rhythm like drums and bass. Quality personal instruction is hard to beat, but if you don't have a local music store or community college that offers lessons or classes, there seems to be an abundance of video that might help. There's even a lot of free stuff online, like this:
http://www.youtube.com/profile?user=BassLessonsOnline&view=videosAs for what bass to get, I guess right now it depends on your budget and how long you plan to stick with it. I'd probably avoid the absolute entry level stuff, but something like a Squier (not Affinity) by Fender P-Bass is about as basic and universal as it gets. Based on various reviews, it seems like most of the low to mid priced basses are decent, if not pretty good. I have never used a small bass practice amp that I thought sounded worth crap, but I am not a bass player and don't keep up with the latest cool stuff, so I may be missing something.
How did I start and what would I go back and change? As I said, I'm not a bass player (although I have one and play it every once in awhile), but I got my musical training through years of piano and violin lessons starting as a little kid on up through high school orchestra and a lot of guitar playing in bands during high school and college. I still play guitar as often as I can. I no longer own a violin and haven't played one in years. I haven't played piano (other than a small midi keyboard) in a long time, but wish I had one and time to play it. If I could go back and do things differently, I'd truly appreciate the opportunity I had and take full advantage of it. I wouldn't whine about being the only guy at a piano recital (or having to quickly swap a coat and tie for baseball uniform to get to my game in time). Or, being one of two high school football players in orchestra. With the exception of the guitar playing, which was my choice, I felt like I was forced to play instruments and it took nearly 20 years from the time I started before I realized the advantage I had gained. So, basically, I had it beat into me from an early age. I'd suggest a shorter method with fewer moments of anxiety and humiliation.
