Well, if you haven't installed linux as of yet, I would suggest that you are getting ahead of yourself
First, you will need to create some empty space for linux to reside in. Use something like "Partition Magic" if you like (me, I prefer to simply install a second hard drive since it costs about the same amount of money)
Second (and this can be part of the RH install), you need to partition the disk so that you at least
have two new partitions available.
Third, do the RH install. It should take care of lilo for you.
Let me know how it works out.