If you don't have 2 GBs of installed RAM, then you need to buy more RAM. You can install:
Maximum Memory 6.0 GB (Actual) 4.0 GB (Apple) Memory Slots 2 - 200-pin PC2-5300 (667MHz) DDR2 SO-DIMM
You can purchase new RAM to replace what you have at:
You need at least 15 GBs of free space on your hard drive. If you don't have that much, then your drive is too full. Either remove data you no longer need, transfer files to an external hard drive then delete them from this drive, or install a larger hard drive.
Freeing Up Space on The Hard Drive
2. You can remove data from your Home folder except for the /Home/Library/ folder.
3. Visit The XLab FAQs and read the FAQ on freeing up space on your hard drive.
4. Also see Freeing space on your Mac OS X startup disk.
5. See Where did my Disk Space go?.
6. See The Storage Display.
You must Empty the Trash in order to recover the space they occupied on the hard drive.
You should consider replacing the drive with a larger one. Check out OWC for drives, tutorials, and toolkits.