Can I split one Ethernet line coming out of my wall into multiple separate lines?

You need something like this, it's an Ethernet switch, it's basically a more sophisticated hub. 10/100Mbit if you don't have faster Internet. No need for setting up, just plug and play. http://amzn.com/B00002EQCW

But talk to your provider first and as for possibility of using this one, they maybe blocking access only for one computer at a time.

I would suggest you purchase a router and plug it to the port. A router has an advantage over a switch in this situation, if your ISP is blocking access to one computer at a time.

A router is probably almost as cheap as a switch these days and far more prevalent among consumer setups. A typical consumer router is actually a router + network switch + wireless access point. So if you already have a wifi router, you would likely be able to use it directly, since most routers should come with 4 ethernet slots as well.