You begin by picking an incentive site and completing a (usually) short and painless one-time online registration form on your selected incentive site. These registration form asks you basic information such as your name, address, email address, telephone number, etc., and other information relating to where and how you would like to get paid (this information is also used for verification purposes.)
I highly recommend that you fill in all fields on the form being as specific and accurate as you can because your membership is subject to the accuracy of the information you provide, crucial information will be verified (see below), using fake information here may subject you to penalties such as eventual sudden unexpected forfeiture of pay and banishment from the site, and last (but not least) this is the information they will use to process your payments.
Once you submit the registration form the incentive site will usually go through some basic verification procedures attempting to verify that you are who you say you are in order to protect their sponsors (the advertisers) from fraud in the form of members creating multiple account aliases (attempting to double-dip on proceeds by submitting fake information.)
The verification usually involves the site sending an email to the email address asking you to click on the provided link in order to verify that it is indeed a valid email address that is yours. Most sites nowadays have also implemented a telephone number verification procedure that will actually call you on the phone number you provided using an automated script that will assign you a numeric code that you must type into an appropriate box on the web site in order to verify that the phone number is valid and yours.
Once you complete the quick registration and painless verification procedures you will be accepted as a full fledged member of the incentive site and you are ready to start earning cash and prizes (Yay!)
If you are interested in receiving your money earned from cash incentive sites fast I would suggest that you sign up with the popular online payment processing services (unless you are happy waiting for a check or money order to be delivered to you via the United States Postal Service ("snail mail".)