"I have a intel based macbook pro 17inches, which came preinstalled with Snow Leopard, its a 500GB hdd, and i want to make a partition for storing my files/data to the next partition so that i can perform a clean installation of lion.
Please guide me as i am a new user of mac."
You _can_ do this on the internal drive.
But -- I would suggest a better way (It will cost you about $75-100, but the money will be well-spent).
Get an external drive of about 1tb in size, partition it into (at least) 2 partitions, and do a "bootable clone" of your current OS (Snow Leopard) to one of the partitions. This can then serve as a semi-permanent "archive" of your old OS. And you can use the _other partition_ to backup your new Lion OS. You will then be "backed up twice".
Here's what you need:
1. One of these:
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias=aps&field-keywords=usb+sata+dock&x=0&y=0
(many items shown, they all work the same, just pick one that's cheap, you don't need to spend more than $20-30)
2. Then, get a "bare hard drive" (1tb size would be good) from any vendor you wish (I like Seagate drives from newegg.com).
3. Then, download the free "CarbonCopyCloner" from:
http://www.bombich.com
You now have the tools to create bootable backups that will be exactly like your internal drive. Once you have this, you will have an quick way to boot up your Mac if for some reason you can't boot from the internal drive.
Once you have the stuff above, do this:
1. Hook the dock up to the Mac, put the drive in it, turn it on (Note: it's not initialized yet, so it won't show up on the desktop)
2. Open "Disk Utility". Select the new (not-yet-initialized) drive on the left
3. On the right, choose "partition". You can set it up with as many as you wish. For a 1tb drive, you might do with either 2 or 3 partitions (one partition to archive your old OS, one to serve as a backup of Lion, and the 3rd as "scratch space")
4. Get the drive partitioned and you will see the new icons on the desktop. Make sure each partition has a distinct name that is DIFFERENT from the internal drive.
5. Launch CarbonCopyCloner. You will see that it has basically 2 "panels" (left and right)
6. In the left panel, select your internal drive.
7. In the right panel, select your backup partition for Snow Leopard
8. Toward the lower right, select the popup that says "maintain a backup (no archiving)"
9. Click ok and enter your password. It will take a little while to complete, because CCC is copying EVERYTHING from your internal to the backup.
10. When done, TEST your backup volume by restarting and invoking the Startup Manager.
10a. Restart, then
10b. As soon as you hear the startup sound, hold down the option key and KEEP HOLDING IT DOWN
10c. In a few seconds, you will see icons of both your internal drive and the cloned backup appear on the desktop
10d. Use the left/right arrow key to move between icons (you can also use the mouse pointer). Select the backup and hit the enter/return key
10e. The computer will now boot from your BACKUP volume (instead of the internal drive). IMPORTANT -- because it's a clone, when you get to the desktop, it should look EXACTLY THE SAME as your internal, and it may be hard to tell the difference. You can verify which volume is the startup drive by choosing "About this Mac" under the Apple menu. (HINT: I give my backups a different desktop picture so I can tell them from my internal drive)
10f. When done, just restart and you will again boot from the internal drive.
You are now "fully backed up" and can install Lion as you wish.
TIP:
You _might_ consider doing a completely "fresh" Lion install to one of the empty partitions on the backup drive. When prompted, migrate your accounts and data from your internal drive.
The advantage is that you will have a fully-functioning Lion drive to check out BEFORE you put Lion onto your internal.
When you want Lion on your internal, just use Disk Utility to erase your internal drive (while booted from the docked drive) and "clone over" Lion to the internal.
Sorry for the length.
But try the suggestions above, I'll bet they work fine for you.