What do I need to do before turning 21 in order to use CSPA?

You cannot prevent it, unless the timing of filing, your age and the length of the petition being pending all together will qualify you for the CSPA.

Nothing you can do except wait and see when petition becomes current you qualify under CSPA

You won't know yet if the CSPA will apply to your case or not. Assuming the I-130 is eventually approved, you're going to take your real age on the day that your priority date becomes current, meaning that a visa in that preference category is available to you. Then you're going to subtract from your real age the amount of time that the I-130 was pending. This will be your CSPA age. If your CSPA age is less than 21, then you have a year in which to pursue residency. If you fail to start the residency application process within a year of a visa first becoming available, then you will lose the protection of CSPA. If, on the other hand, your CSPA age is 21 or over, then you won't be covered by the CSPA and you'll need to wait until your priority date in the 2B category becomes current.