Borrow wi-fi from your neighbor. Viola, free internet.
More seriously: I owned and operated an ISP (and helped operate another, and consulted for others) through most of the '90s and early 00's. The kind of questions you're asking suggest to me that you have some fairly significant knowledge gaps that you're going to need to fill in over the course of the next few years if this is more than a passing fancy for you.
This isn't meant to discourage you, but until you understand the kinds of topics being debated on, say, the NANOG
mailing list (substitute your regional network operators' group as appropriate), you're a very long way from being able to do what you're talking about (building a service that tier-1 providers may be willing to peer with for "free", ie. for the cost of transit; see the link posted earlier regarding peering, although that's a higher-level business/culture topic, and it appears you're still struggling with the basics).
Now, if you just want to build an ISP (dialup in some parts of the world, WISP, CLEC or other facilities-based provider, VPS/hosting, etc...you weren't exactly clear about what you mean by "ISP", which is kind of an overloaded phrase these days), that's much simpler, and will involve a connectivity arrangement with one or more upstream providers; you'll give them money, they'll give you bandwidth. Maybe. If a backhoe doesn't come along, and as long as you don't end up competing with them a little too well.
Nothing is free.
Anyway, I meant to keep this short. I'm sure I and others here would be glad to answer questions (although this really isn't the right audience for a topic like this), but they'll need to be a lot more specific with what it is you're having trouble wrapping your head around. You might want to spend the next few months reading and learning first, though; anything related to CCNA (Cisco Certified Network Associate) certifications are a great way to quickly bootstrap your knowledge and force you to deal with your experience gaps, but any introductory networking books and classes would be useful.
(It's entirely possible that I've misjudged your experience level because of the brevity of your posts; please, do correct me if I have, and accept my apologies if so.)