You can buy Snow Leopard from Apple for $19.99 here:
http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard
You buy the DVD, put it in your MacBook and it will give you the option of installing it. If you need help with the installation, just ask us (I won't describe it here because it is sort of off topic).
What does ram do... I'll explain it in lay man's terms (and I'm sure other posters will say I made mistakes, or forgot something, but this is a basic explanation).
The CPU is fast. Really, really fast.
The CPU has to work on data.
Imagine that all of the data has to be in a bucket. It takes time to put data in a bucket and it takes time to get data out of a bucket.
The fastest buckets are built in to the CPU. They are called registers. They are also the smallest buckets. I mean small like a thimble and as fast as a jet plane. The CPU can only use the data in the registers.
The next bucket is RAM (sometimes just called memory). This bucket is still very fast, but not as fast as the registers. It is much bigger than the registers. This is more like a swimming pool, but only as fast as a Ferrari (which is fast, but not as fast as a jet plane).
The last bucket is the hard drive. This bucket is HUGE. It is huge like a lake.
But it is also a lot slower than RAM; this is a person riding a bicycle.
You can NOT upgrade the registers.
You can buy bigger, faster RAM (imagine going from an inflatable pool to an Olympic pool).
You can buy bigger, faster hard drives (imagine going from lake Tahoe to lake Superior).
If you're still reading this, then good for you.
Here's how the system works: All data has to flow from the Hard Drive, to the RAM, to the registers.
Because the hard drive is so slow, you really only want to get data from it once. After you've got the data into RAM, you want to keep it there.
If you don't have much RAM, then it can't hold much data and you'll have to get things from the hard drive.
If you have a lot of RAM, then you can hold a lot of data before it gets full.
Good luck.