When you will be ranked to member or at least Junior member, you can try to enrol into sigmature campaign that you like, and then they might accept you
Now a days the most important thing for getting accepted in a campaign is the post quality... you might have a sr. member account but if your post quality is poor then it would be hard for you to get accepted in a good campaign.
Correct, Even if you have high rank account with poor post quality then it would be still useless since you cant easily join signature campaigns because sig camp managers are strict to select to their participants especially on quality of post because they dont want to accept those spammer because it would only bring trouble or stain to a signature campaigns.