ignore everything the other guy said, he clearly doesnt know much about solo x's
not trying to be mean, but if you know nothing about what you'll need or what amps etc you need, you are clearly a rookie and need to start out on a MUCH smaller scale.
however, ill tell you what you need just to humor you.
first off, an 18" solo x is gonna run you at least $800 if you get it online. you'll need a HUGE ported box to make it sound good, and you wont be able to fit that in your focus. Solo X's are total power hogs, so not only will you need a couple of extra deep cycle batteries, but you'll also need a high output alternator. 3 batteries would cost you around $400-$500, and the HO alternator would run you at least $600 including labor. then you need 2/0 gauge (thats double zero gauge) power and ground wire, that'll run you about $150-$200 for decent stuff. then the amps are the expensive part. the 18" solo x will take 10,000 watts RMS (not 1,000 like the other guy said). that means you need a HUGE amp. kicker makes an amp called the warhorse which is made specifically for the 18" solo x. they retail for around $10,000. and theyre humungous, so good luck fitting that in your focus too. other companies make amps that would do the job too, but you wont get a decent one for under a couple of grand.
other than that, unless all you'd want to hear is pure bass, you'd need to upgrade your door speakers too. you'd need to buy decent stuff that can take a good amount of power (higher polk, infinity, RE, and kicker lines). they'll run you a few hundred as well. then you'd need a 4-channel amp to put on them so that you can actually hear the music
then you'd need a whole lot of sound deadening so that your trunk doesn't leave the car when the bass drops.
with all that weight you can kiss your great MPG good bye, i'd be surprised if you got over 12 mpg
so all in all, unless you have a few grand lying around to pump into your ford, you might wanna re-consider your first system. good luck in whatever you wanna do though, message me if you have anymore specific questions about stuff.