So, I'm going to buy a new Hard-drive but I just wanted to know if there's anything in particular I should look out for when buying one? In that, are there any compatibility issues I should make sure I take into consideration when buying a new Hard-drive?
If you are adding to a desktop machine, you will be buying a "standard" size of desktop 3½" drive. There is only one common desktop drive physical size. The capacity in GB is a separate matter. If you are adding to a laptop, however, there are several more complex details to consider.
First thing to check is what type of HDD data connection is unused (and hence available) in your computer? Almost all recent machines should have a few SATA connectors available on the mobo. If that is your case, you will be looking for a SATA HDD, plus possibly a SATA data connection cable (unless you have a spare already). BUT if you have an old machine with only IDE connections available, you need an IDE drive. At the same time, check whether you have an unused power supply connector (from the PSU) available for the new HDD. The ones for older IDE drives are quite different from the ones for SATA drives, so check for the right one. If you have none spare, you may need a simple adapter Y-cable that makes 2 power connectors out of 1.
IF your only option is IDE, you should check whether your machine's HDD controllers (usually determined in the BIOS) can handle HDD's over 137 GB. If not, you either need to update the BIOS to a newer version with "48-bit LBA Support" or limit yourself to a smaller new HDD. This is NOT an issue for SATA drives - ALL SATA systems can support larger HDD's.
When buying SATA HDD's these days you have a choice of SATA 3.0 Gb/s (aka SATA II) or the newer SATA 6.0 Gb/s units. No matter which type of SATA controller system your mobo may have, NO mechanical (spinning disk) HDD can exceed the abilities of the SATA 3.0 Gb/s connection system. The only "drive" devices that MAY need the faster new standard are the SSD's. So, if you have to pay extra for a SATA 6.0 Gb/s mechanical HDD, it's not worth the extra money because you will get NO additional performance from it.
If you are buying / installing a SATA drive, do NOT get confused with discussions of "Secondary Drives" and "Jumper Settings". Those are issues for IDE drive systems only. In fact, as a rule you should NOT be changing any jumpers on a new SATA HDD because you can actually cause yourself trouble by doing that. BUT if you do buy / install an IDE HDD, you must set up jumpers and cable connections correctly. I won't go into details here. If that is your situation, post your details and I can advise.
Be aware that NO new drive can be used right away just because you plug it it. There is a two-step process to prepare your new HDD for use by your OS. First you have to establish one (or more, as you choose) Partition on the physical unit. This defines a particular region of the HDD space to be used as one logical "drive" with its own letter name, and also writes at the very beginning of the physical unit a small Partition Table file with this info for the OS to find. Then you must Format the new Partition - that is, create the file-tracking system (a set of files) that will be used on this "drive" (Partition). These can be done either using a free utility from the HDD maker, or using Disk Management built into Windows. VERY often these two steps are combined into one in an easy-to-use Wizard interface. If you need more guidance here, just ask and we'll help.