So people keep telling me to contribute to open source projects on github, so that employers get impressed. Any idea how to start doing that :D

Step 1 done, will work on other steps

Really, I'd say the key is to look around for projects that you like/use/are interested in and see where you can find their source code & issue tracker.

Often these days, that'll be GitHub. Then, when you've found a project, look around at their source code 'till you get a feeling that you understand how it works. If you don't, research the languages/tools/techniques used. If it's too much and you're overwhelmed, ask for help or look for another project. Once you've found a project you're comfortable with, look at their issue list and/or pick an aspect that you feel like you can improve. Code it and create a PR. Rince and repeat...