Well first of all you have to make sure you have to download and burn Win7 RC disk, remember to copy the code microsoft gives you then make sure you have an empty hard disk or partition; if you dont, you will have to either format the whole hard drive or make a partition. WARNING: you will loose any data in your drive if you format a partition, make a partition, or format the whole disk if it isn´t parted; make sure you have a copy of your files.
You can format from W7 RC cd, make sure your motherboard´s bios is set to boot from CD before booting from hard drive and it will present you a screen were you will decide wich hard drive or partition you want to format, if obviously you only have one then you can only choose and format that one; hit enter and chose to format as NTFS:
Depending on the size and amount of information on the drive/partition go to you microvawe, toss in some popcorn, go watch a movie, when you come back it should be asking for the code (the code microsoft gave to you remember? they should give you one when you sign up for RC testing; check your mail or spam box or junkbox... you´ll find it!), then fill in your name and other stuff you can only know yourself, add username and password, etc. and voilá! you´re ready to start nipping and tucking all you want!
Usually different partitions have different letters and show the space on disk or partition, that´s how you can tell wich one you are formatting; any more questions feel free to ask around.
PS2: You cant make partitions from Win 7 boot cd, you can only format drives or partitions who already exist so if you plan on splitting your hard drive you may have to download a partitioner cd or partition from inside vista if you have it or use other similar software.