Here, I drew you a nice diagram. Its late, so it was rushed.
Anyways, Im not going to repeat whats been said, but just add on.
Headshells generally come with turntables, if you get a used one, you might not. In that case, they arn't too expensive, so you can pick one up somewhere. Make sure it will fit onto your tonearm. Most brands of turntables have specific headshells to use. (Numark uses their own headshells, Technics theirs, etc.)
You connect the cartridge to the headshell with wires and screws (see the wires, they are color coded so its easy to do assemble). The screws so you can hold the daggum thing on, and the wires for output (into the tonearm, into the table, into the mixer, into the speaker, bam, theres your music). You can put weights (come with either headshell or cartridge) between the top of the cartridge and the headshell, held by the screws for more weight and force onto the record to decrease skipping.
The cartridge is easy to wire up to the headshell, and easy to screw into place. People in the DJ world and community use cartridge, needle, and stylus interchangeably. They are basiclly refering to the the entire setup, unless specifically talking about one or the other.
To start, you need an entire cartridge, and they should come with needles as well. Dont buy used ones,
period. I dont even want to explain why. Just dont. They are easy to hook up, probably 15 minutes tops for each one. You will have spare parts (screws, wires, etc), so dont be worried that you have stuff left over.
The stylus slids in and out of the cartridge, and they should be replaced every 6-8 months (i would say anyways). You can buy them seperate and they cost much less. There is no need to go out and buy another cart and needle, unless you want another.
A very important part about all this is once you get new stuff, you
MUST break in the records. Run a search about this on the search engine.
Oh, and if you have two tables, you will need two of everything.
The most important thing is to remember that the more expensive, generally the nicer they are. Dont go get the most expensive, cause those are not for DJing, or scratching. But the cheap stuff is absolute garbage. Ask us before you buy.
Most guys who scratch get either Shure or Stanton. Uhh, im not biased or anything

, but Shure is the best, and when properly set up on any table, is the most ideal for scratching and mixing.
If you want to do more mixing, get Ortofon, or the more expensive stuff that I cant spell.
I feel like I left something out... but its late, its been a long week, and ive got another long day ahead of me.