Re a new OS X and adding Memory for my laptop

I am new to this site and I hope I am phrasing this correctly. I have a Mac Book with
OS 10.5.8 (Leopard) and with 1 GB 667 MHz Memory. I'd like to update the OS and increase memory so I can add a longer life to this laptop. How would I find the latest possible upgrades and which should I do first. Thanks.

Go to RAM Memory Upgrade: Dell, Mac, Apple, HP, Compaq. USB drives, SSD at Crucial.com and input you computer info in their memory finder tool and it will tell you.