Solobarics Kicker small and bring more bass, Fosgate MTX is cheap and good quality than Sony I wouldnt recommend them dont have good subs.

when i buy a system what do look for? watts, size, and also whats a decent brand, like the cheapest that will do good.

No, how much power a sub will handle has nothing to do with picking a sub, unless you've already bought a 1,000 watt amplifier.

When picking a sub, decide on the size you want first. A 10" sub will play ~95% as loud as a 12" sub, of the same brand and type, just not as deep. It also won't play as loud at the lower frequencies.

Once you've decided on the size, look for the most efficient sub of that size, unless buying buying monster amps, plus $600-1,000 alternators is your cup of tea. For every 3db loss of efficiency, you'll need twice the power, to get the same volume.

edit: That's the reason almost all winning competitive car audio systems use 10" subs (they give more 10's massive amounts of power), but almost everyone on the street has 12's (they are slightly more efficient than 10's, and can play lower, louder).
Also, the cheapest decent sub, in my opinion, is Kenwood, although if you give any decent sub at least 3/4 of the amount of RMS power it's rated for, it will boom.