It is very easy, however there are a few things to note.
1) You'll want to make sure that the hard drive you get to replace the bad one has the same capacity, RPM speed, and disk type (SATA vs SAS, etc.), and you'll also want to make sure it's a supported drive by your vendor; in this case Dell.
2) You'll want to know the exact disk that failed (should be obvious with blinking status lights on the front or somewhere on the server) as well as which slot/position it's in in your RAID setup in the RAID bios.
Once you know these things you can, as you've already done, order a new working hard drive as a replacement and pull out the bad drive and put the new one in it's place. This again assumes you have a backplane in your server connected to the hardware RAID controller. If you don't have a backplane and it's just a standalone flimsy RAID controller, then you can still remove the disk, you'll just have to disconnect it from the controller a little more carefully.
You can monitor the status of the rebuild by entering the RAID bios upon startup. Dell PowerEdge's typically use PERC controllers so to get into it you will want to look for the prompt when you first start your server that says "Press Ctrl+R". Simply press this keyboard shortcut and you will load into the RAID bios. Once you're in there, you can monitor the status of the rebuild and see a percentage as well as other alerts.
You can also install the Dell OpenManage tools to help you monitor RAID from within Windows. Give them a call as suggested in one of the comments above and ask them about that.
Let me know if you need further help, but don't be scared by this process as it's very straightforward.