How do I update my Macbook from OS X 10.5.8 to one that'll run programs?

I bought my Macbook Pro in 2009 and got a good few years of use out of it before I started having chronic problems with the battery life. I got a new computer in 2012, at which point I retired my Macbook into storage, where it stayed until earlier this morning.

I'm trying to get it running again so it can be used around the house, where it'll be plugged in at all times. The issue is, I'm not able to do much on it currently because the OS is so far out of date, and I don't have a whole lot of GB to spare. It's a 2009 Macbook Pro with Mac OS X version 10.5.8 currently. It has 2 GB of memory, and a 2.26 MHz processor speed.

What's my best option?

Thanks!