How are people chosen for world championship?

Top 3 seeds from NALCS, EULCS, LPL and OGN, Top 2 seeds from GPL, Top 2 seeds from the IWC tournament.

It's really simple. Assuming you don't directly picked up by a pro team:

1. Make a team 5v5 Team and make it to Challenger.
2. Finish in the top 20 before the deadline to qualify for the Challenger Series.
3. Survive the Play In phase to make it to the Bracket stage to get points. Note: There are 2 Challenger series per split.
4. Finish top 6 during to qualify for the Challenger Series playoffs.
5. Finish top 3 in the playoffs to qualify for the Promotion Tournament.
6. Defeat an LCS team in a Bo5 to qualify for LCS.
7. Finish top 6 in LCS regular season to qualify for the LCS playoffs.
8. Finish top 3 LCS at the end of the Summer split to qualify for Worlds. Note: LCS will most likely change to a Circuit Point system in 2015 so this part might change.

Easy enough.