Depending on the companies you are signed up with, it will vary. Some companies offer "Self-assign" jobs on their job boards. If you see these, you need to make sure you fit the qualifications for the job (these are likely to be age, shopper rating by that company and whether you have shopped the location longer ago than a certain period of time). All you need to do is to click on the acceptance of the job and it is yours to do as posted unless you did not fit the qualifications. For other jobs you will need to request them from the job board of the company and based on your profile and prior experience may or may not be awarded the jobs. When you see posts on the job boards of public forums, such as this, or on job boards such as Jobslinger.com, the method for sign up for the job is usually spelled out pretty clearly.
It is usually most difficult for new shoppers to get assigned jobs with companies for whom they have not previously worked. So it is to your advantage to take a few 'self assign' shops if possible from a company to begin building your rating and reputation with them.