Joining a pledge
It is simple to take part in a pledge (and pledges are hosted here). Simply read the information and decide if it something that you are willing to do, click the pledge button and sign up. Pledges are started by a 'pledge holder' who should give enough information for you to decide whether you want to join the pledge or not. You can 'unpledge' at any time.
For some pledges, you can start the action immediately, the information should make this clear. In general DON'T UNDERTAKE the action until the correct number of participants have been reached AND THE PLEDGEES HAVE BEEN CONTACTED to say the action is beginning. Especially in the case of acts of civil disobedience, the idea is for strength to come from the numbers of people pledging to take an action at the same time. It's possible that if a big enough threat of taking action is reached, that those the pledge wishes to influence will listen to the will of the people. There may also be a period of verifying that people are in a position to undertake the pledge.
If you are keen to do something to support the pledge further then it is helpful to let other people know about the pledge in case they are interested in joining up.
Always read the legal information, though do be reassured that there is nothing illegal about pledging to undertake an action. As the pledge grows and when the time comes to undertake a pledge the pledge initiator will be in touch with updates and information.