How to quickly get started using RAID?

[Mitch]

I would recommend using a hardware RAID controller for duplicating your OS with RAID 1.
With a hardware controller you have these benefits:
- OS doesn't know about the underlying disks including boot partition.
- You can remove one disk and still boot up (in theory you could do this with MD raid in Linux, but not Windows).
- Independent from motherboard (fakeRAID e.g. Intel Rapid Storage Technology)
- Less problems after BIOS reset e.g. from (fake)RAID mode back to AHCI.

Cons:
- Checking the disk health with SMART could be different per (controller) vendor.
- More expensive than fakeRAID (motherboard).
- You need a SAS SFF to SATA cable.

For data storage software raid is the easier, cheaper and you have a lot of options under Linux e.g. MD, BTRFS or OpenZFS.

And yes, you need the same size disks (i think it uses the smallest size).
(For HDDs: don't buy SMR (Shingled) hard disks.)