1. Go to your Utilities folder, find Disk Utility, and then select one of the HDs you want to use for the raid, select RAID, select what level of RAID you want (striped/mirrored) set, and then follow instructions (Aka draggin' discs/volumes into Disk Utility's Box). Backup before you do anything.
2. Generally scratch drives are only used for data - the OS and Application reside on a separate disc.
There are several levels of RAID. JBOD (Concaternated?) stands for "just a bunch of discs", in which the two discs just get added on top of each other, with no speed increase or redundancy. RAID-0 is the speediest array type the data is "striped" along the hard drives - practically doubles the read and write times. RAID-1 doesn't increase write times, but if one drive fails, you still have a data. RAID-0, one has a hard drive fail, all data is lost and one has to resort to backup. Note the RAID-1 is not backup, merely redundant.
Those are the software levels of RAID that OSX can provide, IIRC (Besides 10 and 01, which the numbers correspond to the levels I just mentioned). TO get more advanced RAID levels (RAID 5, 6, etc.) one needs a RAID card... etc etc. By then its a question of the amount of investment.
I would recommend reading Wikipedia's article on RAID, specifically levels zero and one as those are the ones OSX provides free of charge.