I am assuming you have a Mac Pro desktop (Big Silver-looking tower) and not a MacBook Pro. You may not be suprised how many people make the mistake of confusing the two and posting in the wrong forum. If you Do have a Macbook pro, consider posting in that forum. With that out of the way, it's easy. Your mac pro desktop has 4 drive bays, which can hold 4 hard drives with sata II interface. I've heard that you can put 8 tb drives in there, but I can't say for sure. You are welcome to try, though. back to your question. Here's what you do: go buy a reliable brand or model of hard drive in the size you want. I bought Western digital and a seagate. both have been fine for now. Buy 3 of them. take them out of their package. Shut down your Mac Pro desktop, take off the side panel . Wait a bit. below the optical drive are the 4 hard drive bays. assuming they are not locked, put your fingers under and pull them out one at a time towards you . install the hard drive into the drive sled with a #2 phillips screwdriver, with the power and sata plug towards the rear . Push the now filled sled back into the slot, al the way back in. You can see if it went all the way back or not. After getting all the sleds back in, put the side cover on. reboot your machine. go to disk utility and format your new hard drives appropriately. You are now done. Might I suggest that while you're going about getting more hard drive space, you should also get a backup system of some kind and use it. Hard drives may fail without warning, and if you haven't done a backup of your important stuff, you could be in a lot of trouble. I use an Airport time capsule which is easy to set up and use. backups are automatic as well.
just my 10 cents
John B