If your factory Radio has pre-outs then you can connect to any using RCA plugs. If your factory radio only has speaker out wires then you need an amp with High in's. (speaker wire)
Best Amp local is close to receiver for performance but under the seat for security.
Sub amp would could use the Pre out's from amp or other way depending on location. You could get a RCA Splitter from Radio Shack for a couple buck if you don't have the RCA outs.
2 ohms is twice as loud on half the volume. Most speakers are 4 ohm factory. better stick to 4 Ohm.
If you have all 2 ohm speakers you could do it that way and if amp is 2 ohm stable. 4 should work great for most applications.
Amp will need 18 Gauge or better. Ground Radio and amp to chasis same place. Seat belt bolts are great. Keep that ground cable 18 Gauge or smaller and less than 2 ft for best results.
Remove battery cable before doing any work. and good luck.