The word ISP is a pretty broad term, I'd assume you are talking about providing the capability to provide broadband access to residential or business customers?
If you have space in a carrier nuetral facility it is relatively easy to provide high speed acess to others. To obtain IP's you would either get them from your upstream (for single homed bandwidth) or from ARIN (multi-homed with ASN number). Then you need some routing equipment and you are good to go. You would contact another telco to run the connection from the datacenter to the client, they charge alot less for connections if there is no web access needed on them.
If you have to ask here I would suggest not trying. Expecting people to spoon feed you the information that you want to base your core business on is a sign that you lack the knowledge and probably the skills to run/start this business successfully.
I think you might be better off searching google and researching for yourself on how to perform this business, it will show some initiative on your part.
Please do not take this as me being mean but as me giving you a dose of reality.
I got into this business getting hired on at a ISP back in the late 90's as a server admin that started up from nothing in a small town.
It's probably a little easier in some ways now, but I certainly wouldn't wish it upon anyone to try breaking into the market and going through all the headaches we did.
As a small startup ISP you're constantly competing with the likes of AOL, Netscape, AT&T, NetZero, Broadband, etc. Not easy, but in our case we were able to cater to the local market and provide a high level of personal support the big guys couldn't and even then our numbers remained pretty static after a couple years blocking any further growth before we sold out to the local telco.
As others have said I'd re-evaluate your interest in this as a business, it's just too much of a commodity these days and you'd really have to bring some tremendous unique or valuable to the table to make a dent.
There's also a lot of legal issues up in the air that could block or make the prospect of selling services like broadband very expensive due to legislation being changed on whether phone or cable companies are forced to allow subsidized use of their lines.
There are quite a few extremely successful small ISP's out there, if you can cater to the local market with a localized brand and customer service (get involved in local events, radio stations and the like). Nothing is impossible
Aye, by no means is anything ever impossible, but where's an entrepreneurs money best invested? In most cases it's definately not in a market that's already saturated and commoditized. You really want that first mover advantage with nice margin's and fresh prospects.
You can scale fairly well as a small local startup ISP too, but I still think there's better prospects for a return on investment that you could probably bootstrap better.