How to split a ethernet jack for two computers
 

We only have three ethernet jacks in our office area, but have recently added a fourth desk as our business grows.

All jacks connect back through a patch panel to a router plugged into the T1 modem.

If we wanted to split the third jack for two computers while waiting for the network company to install new plugs about 45 days from now, would we use a basic switch and plug the network plug into the uplink port, or something else?

Yes, that's all you should need to do.

Will the patch cable between the rj-45 on the wall and the new switch uplink port need to be crossover cable?

Most recent switches use auto-detect ports that remove the need for a crossover cable. It should say on the box, possibly listed as something like "Auto MDI/MDX detection". It's also likely the uplink port is going to be married to one of the local ports such that you use one or the other, with each wired in reverse from the other or it'll just be auto-detect.

CAT 5 can be split into 2 lines by using the 2 unused pairs. however i second the switch.

Do not use a crossover cable. Those are only used for CPU-to-CPU connections that don't involve a switch/router.

Some older switches, DO need a crossover cable to go switch to switch, but most are auto sensing or have a mechanical lever to switch the wires internally to make it into a crossover.

An uplink port serves the same purpose as a crossover cable (which is to connect one end's TX pair to the other end's RX pair). If you want to connect two hubs or switches, use either a normal cable between a normal port of one and an uplink port of the other or a crossover cable between two normal ports.