I'll try to keep this as short as possible, but I have to warn you, modern stereo technology has MANY options and configurations. Part of the fun (and the frustration) is fiddling with the options to get the sound you like.
If you are hooking up two subs to a single-channel amp, you have two basic wiring schemes - serial and parallel. Here's a picture of a serial connection: Attachment 166262
Think of a serial connection as a daisy chain of speakers.
Here's a picture of a parallel connection: Attachment 166263
Parallel connections kind of "stack" instead of chain together.
And here's the website where I pulled these images. It gives a much more thorough explanation than I'm giving you, along with additional calculation tools. Series Parallel Speaker Impedance
So what's the difference? In general, serial connections aren't as loud and make the amp run cooler. Parallel connections are louder and make the amp run hotter. Just like your motor, heat is the enemy. High quality amps can handle properly configured parallel connections. Cheap ones will fry. There is a very important technical quality about your speakers that you must consider - impedance. It is measured in ohms. (I started to write the rules for calculating the serial vs. parallel impedance, but stopped because I'm not sure you want to know. Plus it's all on that other website.)
My final recommendation is to check out a site like Crutchfield.com. That company has been in the car stereo business for a LONG time. I've purchased a bunch of stuff from them over the years and they have all sorts of kits and experts to take your calls. They aren't the cheapest, but the support they offer can be worth the little bit extra when you aren't familiar with this stuff. (And no, I have no affiliation to them.) Of course there are other great sites, not to mention solid local retailers. Like I said before - talk to the experts. They don't mind. I put myself through college selling and installing this stuff. I was always happy to explain this to enthusiastic newcomers.