Everyone can join a signature campaign, unless you're marked as untrusted then there would be a problem, but first you'd need a higher rank that can be obtainable by posting and gaining activity points every two weeks (You can find everything you need to know about that in a newbie readme). Once you rank up, you can head over to the marketplace>service section and look fora  asignature campaigns that you'd like to join. There are a lot with may different terms and payment options, so try to write down what are those sig campaigns offering and you'll find the one that suites you the most. Good luck.