Are you planning on being an originals band or a covers band? Not that I'm Mr. Voice of Experience, but from what I can tell, covers bands have an easier time finding "success" in the sense of getting a regular round of gigs at local bars and parties. If "success" means fame and fortune, though, to get beyond the local circuit you'll need originals. As others have said, there is no guarantee of big-time success, so go ahead and dream big but don't build your life around the assumption that you'll be millionaires someday. They don't need to be mutually exclusive, though -- bands like the Who and Rolling Stones started off playing covers of blues and Motown tunes until they had their originals ready to launch.
A thought that is based on things I read on this forum more than on experience, but I'll throw it out there; one of the things that seems to me to be a hurdle for originals bands is quantity of material. A covers band can learn the top 40 and have three hours of show easy; a lot of originals bands, especially early on, seem to have only eight or ten songs so they end up splitting the showbill with three or four other original bands, which in turn makes it harder for them to stand out and build a following of their own. So if you want to play all originals, set yourself an aggressive but realistic schedule to get stuff written - don't muddle over just one tune for a month, but get a writing cycle going so that you can have a one-hour set of 12-15 songs ready to play out as soon as possible and build more from there till you can match a covers band's three hour show. Also, work on writing things that audiences can connect to rather than stuff that's only interesting to you.
Beyond that, be mature, work hard, practice diligently, communicate with each other freely, and remember that it is important to market yourselves and to network. You need to cultivate good relationships with venues, and venues want acts who draw paying customers, so you need to cultivate a fan base as well.