Update/new memory and hard drive

Hello,

I have a 2008 MacBook 2.1GHz Intel Core 2 Duo (13-inch White) MB402LL/A. I continually recieve a message that my "startup disk is almost full" and my programs freeze or "quit unexpectedly". I took my computer in to the Genius Bar to see if they can help me with anything, but the "Genius" only told me that my storage is full (which I had already told him I knew) and I can either buy a new computer or I can easily modify/upgrade the computer I currently have. He went to the Crucial.com website and showed me two items which he recommeded I get if I wanted to update my computer. The first was to update the RAM memory 4GB kit (2GBx2) DDR2 PC2-5300 • CL=5 • Unbuffered • NON-ECC • DDR2-667 • 1.8V • 256Meg x 64 • • Part #: CT3265302 to deal with the "slow computer problem" and the second was to update the internal hard drive Western Digital - Scorpio Blue 500GB Internal Serial ATA Hard Drive for Laptops for more file storage. My computer is currently not backed up, so I also purchased a Seagate - FreeAgent GoFlex 1TB External USB 2.0/3.0 Portable Hard Drive. I do not know what my next steps are. I have seen videos and read instruction on how to install the RAM Memory and the internal hard drive, which seems easy and do-able. But my biggest issue is once I put in the new internal hard drive how do I get my stuff back? Would all my software, applications, documents, files, pictures, music, etc. be gone? I have very little computer knowledge so any information, advice, tips would help.

Thanks!🙂