You'll need a minimum of 2GB for Lion, and a number of people have found performance unsatisfactory with 2GB, which was fixed by going to 4GB.
While you could go to the Apple store, you would be paying more than you need to in order to get quality memory.
Two sources are generally recommended, which are Crucial at http://www.crucial.com and OWC at http://www.macsales. com. Both have quality memory with lifetime warranties. Crucial usually has a 5% off coupon on retailmenot.com and has free shipping. I've bought from both over the years and can recommend them. Crucial has a memory selector to help get the right memory, and OWC has tested mac systems extensively, and can tell you the maximum memory your system can use, which may be more than published Apple specs.
You should have information in your user guide that tells you how to change the memory. If not, there are video's at http://eshop.macsales.com/installvideos/ and step by step instructions at http://www.ifixit.com
PS. make sure your macbook has an Intel Core 2 Duo processor and not a Core Duo processor. Lion requires a Core 2 Duo or later processor. You can check by clicking the apple on the left of the menu bar, and about this mac.