Only validators can add blocks to the chain. To become a validator, you need two things:
1. Become a delegate - to become one you have to burn 1000 SEM. those burned SEM are gone for ever and you can't get them back.
2. Gather enough votes for your delegate. Only the top 100 voted delegates become validators.

Currently about 33k votes are required to get into the top 100, but this number increases over time. If you don't have enough SEM to vote for yourself, you will need other people to vote for you. To encourage others to vote for you, you can share the block rewards with them - in other words, become a pool. Once someone votes for you, the amount of SEM he voted is locked, however, at any given time he can unvote you and unlock it.
Here is a list of the current operating pools and their fees: https://bitcointalk.org/index.php?topic=2159012.msg29319963#msg29319963.
If you need help setting up a pool, or have any more questions you are welcome to join the Discord channel.