First of all you are not going to make a fortune doing online surveys. Any website which promises thousands a month is lying. The rewards vary depending on the company. Some will offer cash rewards or points which can be converted to cash, others pay in gift vouchers and some will only give entries into prize draws. Nearly all of them have a minimum amount that you have to reach before you can redeem your earnings which is why it can take a while before you start to see any money. Reputable survey companies are free to join so be wary of any that ask for a signing up fee. There are places which for a charge will give you a list of websites that you can become a member of, however, you can find the same names for free by using a search engine.
A great website to use when starting out with surveys is Survey Police. They have a directory of panels which have been verified by them, a forum and lots of advice. It's a good idea to do some research before signing up with anybody. At the very least you should know where the company is based, what incentives you will get, how you are paid and if it's an international site in what currency. Some companies will accept people from all over the world, others are country specific. If you are joining lots of places, it might be worth setting up a Paypal account as this is a common method for payment transfer.
It's really up to you how many panels you join. If it's several it's a good idea to set up a spreadsheet or keep a notebook of all the surveys you have completed, how much they were worth and if you have been credited with the right amount. Most survey opportunities are sent via email so keep all the ones you have done in case of any queries. Good survey companies won't send you spam (I've never had that problem) however consider setting up an alternative email address just for surveys invitations..
When filling in a survey, answer honestly and take your time. If the company thinks you haven't completed the survey properly you may not get your reward for it. Be prepared to be ineligible for some surveys. The amount of times this happens depends on the company. You will find that some send you surveys that you hardly ever qualify for while others you will rarely have a problem with. It's trial and error finding the good places.
Personally, I like doing surveys but at times it can be frustrating. I have recently left a company (I won't mention names) who took six weeks to send me a confirmation email and despite the dozens of invitations I receive I have only ever qualified for two of their studies. I have also been kicked out when I was three quarters of the way through a survey which was the final straw. However, I am also a member of reliable ones who send interesting surveys with decent incentives. There are plenty of companies around; just do your homework and don't expect to earn a fortune!