Searching “contributing to open source” throws  About 30,100,000 results (0.56 seconds) 

Now the real question is being an Open Source evangelist, what/how should one proceed when talking about same?

I am still from Old school & believe contribution to Open Source comes from inside hence:

1. Telling someone to use Open Source because its virus free is absolute BS.

2. Telling someone to use Open Source because closed source software is bad, is another BS.

3. Telling someone contribute & get a chance to visit across world is another BS.

These things are secondary & if someone gets motivated only after knowing these then its not contribution but need fulfillment.

Open Source is all about choice not a circus where you feed elephant & they will dance.

If you want to  be an Open Source Evangelist in the purest form, please go read these 3 books before grabbing a mic or getting to speak at any conference.

1. Art of Community 

The critically acclaimed The Art of Community by Jono Bacon and published by O’Reilly brings together over a decade of experience in growing, empowering, and leading communities to success.

2. Open Advice

Open Advice is a knowledge collection from a wide variety of Free Software projects. It answers the question what 42 prominent contributors would have liked to know when they started so you can get a head-start no matter how and where you contribute.

3.  i want 2 do project. tell me wat 2 do.

This book has been written to help students and professionals as well to work on free and open source software (F/OSS) projects. These software projects are released under a freely distributable license, which allow you to use, copy, make changes, and distribute the software. 

Apart from this it will be great to read Eric S Raymond books if you want to contribute to Open Source:

How to become a hacker

How to ask questions the smart way