A Solid State Drive can seriously boost an old MacBook's performance, speeding up the time it takes to launch programs, complete tasks, and render large files.
Here's how I upgraded my 2009 MacBook Pro to an SSD and made it feel brand new. Any MacBook Pro from 2012 and earlier can be upgraded as well. (Newer models have the hard drive soldered in and can not be modded.)
To clone to drive, launch SuperDuper, which is completely free. Set it up to clone the Macintosh HD to your new drive. Select Backup - all files. Click Copy Now, and take a break. This could take up to a few hours, depending on the size of your drive.
You're set! Turn on your computer, and give it a few moments while it gets acquainted with your new drive. If you did everything correctly, your computer will launch as usual, but it will perform a whole lot better.