You have to meet certain security criteria.
To start the process all you need to do is fill out the carrier profile in the c-tpat portal - it will get you access into it.
after that you need to fill out the security profile in the c-tpat portal - if you have any questions about it you can post them in this forum - I'm sure there's loads of other people that went through it already
Once your security profile is accepted, CBP officers working on C-TPAT will visit your (or one of your) terminals/warehouses/offices for a validation. They will be checking how closely your security profile matches what you actually do.
If you pass, you are then c-tpat certified. You will have a re-validation once every three years or so after that.