Anyone here have experience with RAID

I'm building myself a PC for xmas, its my first time ever doing so...

basically the motherboard i bought has a built in raid controller that is capable of doing RAID 0,1,5,10

I have 3 1TB hard drives that i want to set up as a RAID 5 array

the problem is that one of the drives is already formatted NTFS with ~900GB on it

first off what would be the best way to approach the situation. Should i take the two empty drives and set them up as a RAID 5 array first. And then add the third disk with data to the array. Then would the raid controller reconfigure the RAID array so that the array now has a final size of 2 TB (n drives minus one for RAID 5) and contain my 900GB of data

or will i need to take the two blank drives set them up as a RAID 5 then copy over my 900GB of data, and then reformat the third disk and add it to the array?

I have no experience with RAID arrays and so I have no clue what to expect.

How do i even go about setting up a RAID array. Would i need to boot into the bios and configure the array there?

LASTly, because of the sheer size of the data in my possession, i was thinking that I should install the operating system onto a separate disk so that if worse comes to worse and i have to reformat/reinstall my operating system, i will be able to do so without needing to backup my data, which i don't have enough external devices to do so....I also expect my data to grow into the upper 1200GB range once i get everything set up. (my college campus has a local peer to peer network in which we all share everything we have...so downloading ~50GB of movies is completely feasible and actually takes ~2 days)

I'm looking for any sort of input here as well as some general help/guidelines