Sneezed, welcome to the jungle!
By now you may be somewhat overwhelmed by all the inputs you're receiving. Note that you've got a lot of folks on here with massive amounts of time in the bass game, so a lot of great suggestions have came your way already...
.... now of course we aren't in your shoes, so none of us know what your circumstances are, such as your age, level of current music education, school and/or day to day workload, ability to easily get to a music store with a reasonable array of gear, whether you have a decent mentor to help get you started, financial situation, true motivation, etc... the list goes on.
What I can say is that if I could go back in time, and had all the things available today that didn't exist for me in my youth, I'd track somewhere along this path (and yes, I'll be parroting what some have already said):
1-I'd ask myself if I'm truly ready, motivated, and have the time to dive into something that may or may not be something I'd stick with. This is a road of true dedication. This is the single most important thing to work out before the big dive.
2-find a good bass teacher, one with a lot of experience in bringing new players with no experience into the bass/music scene. Preferably one with a few different bass models to use prior to purchase. If you sign up to work with them on a routine basis, some may even have a short-term loaner, doesn't hurt to ask. Part of what they should be able to do is guide you to a reasonable and functional first purchase. This to me is the next real step, and this will cost money, but remember, good education is valuable.
3-Within a fairly short time frame you should be able to gauge whether you really want to stick to it or not. If so, by now you should be prepared to actively search out a bass. I echo what others have said regarding use of an available guitar amp to start. I oppose the notion of buying a large gig-grade amp at this point, because you may find (as countless others have) that this really isn't your cup of tea after all. If this transpires and you've shucked out a large amount of money for a new amp you won't use, well, there's a whole bunch of folks out there that would be glad to take your new amp off you at a substantial loss.
4-After your "burn-in" period, anywhere from weeks to a few months, you should be able to make the call on whether to get a gig-grade amp. The type of music, age/maturity, and experience of potential band mates will help dictate what type and size of amp to flow towards.
5-expect the unexpected in the music scene, and do your best to drive steadily through all the twists, turns, and obstacles you'll surely encounter on this journey. It is not an easy path, but for many of us it IS the path, one we don't want to get off of.
All the best to ya!
The Fool