What is the best way to setup internet for multiple offices ?

I would like to setup cable internet access for 12 to 15 offices that are spread out over 3 floors of a 100 year old Victorian style brick home that has been converted into office space.
Each office is a separate company, but I would like to offer them free internet.

How can I make this happen ? I would like to do it myself ?

Also can I limit the usage so that one person can not take up all the bandwidth?

I know your trying to provide better services for your customers, but you should consider the following first:

The legal side of things. You could be help responsible if someone hacks into your network and steals data from any of the companies. You should consult a lawyer first. If you provide free access you'll want each company to sign a waiver.

You'll have to consider the amount of bandwidth you would need. Best thing to do is to contact a broadband service provider. Partial T1 lines are about $400 per month. Anything you do whether wireless or wired is going to involve connecting devices by cabling at some point. You'll want to invest in a good firewall.

Also consider that the companies will become dependant on this service and if there are problems they will complain big time. This is where T1 service comes into play. It is a dedicated line meaning you are guaranteed service and maintenance based on your plan.

You should have one switch per floor. If you can afford Cisco switchs with Fiber ports (to connect the switches together) that would be the best solution. Also consider a PIX firewall. The Cisco switches will allow VLAN'ing so you can control bandwidth for each office. This is typical of managed switches.

Cost of a decent network can be about $10,000+ with a monthly cost of $400-$1,000.

Use a wired router and get yourself some cat5 cable making tools and cable. This will be more secure than wireless too. The router will take care of the bandwidth.

I personally setup a local private school in a similar way. You will need a true server, with Windows 2003 and a proxy server. I tok the time to run CAT5e to all the computers. This is expensive but much more secure than wifi. 3Com makes an excellent switch that will tie you into your server. The software proxy server will allow you to control access from inside and outside of your system.
What you are attempting is very risky. If you have a breech the entire LAN will suffer.
Another option my be to contact your DSL or cable provider and see if they can offer you a package deal for an IP address for each office. They then could simply install a router per office .

the easiest is to divide by floor. Each floor with a router (4-port or 8-port) and each one with a dsl or cable modem. You can try to connect all three together (using unlink) but the bandwith will be divided by the people that connect to the internet. With only a 3mbps connection, it will slow down with 10 people on it to possible the speed of a 56k modem. Another way is to setup switches (12-port or 24-port) and a server. Connect the dsl or cable modem to the server, and setup as a proxy server. Then allow only a few people online, or everyone (according what you want). More people that are online, it will slow down.
The router configuration will be much slower transfering data between computers, the switch will be much faster for that purpose. If you don't know about networking, you need someone that does to set it up.