It's a very broad subject as you may already be finding out! You might wish to decide whether you want to promote something that gives you commission for a "one-off" sale, Dependant on what that product is the purchase price can run from a few £ to thousands. Many schemes will require you to build specific websites with what is known as "funnel" pages, you drive the traffic and hopefully reap the rewards.
The other (and long term) more lucrative type of affiliate marketing is to find a product that has recurring fees AND pays a percentage of the fees to you every time the purchaser pays the next instalment. If you are in the UK and promote a product from a vendor outside the UK, check that you can get commissions paid by direct transfer (say PayPal) because if you get paid by cheque from the US for example, the banks fees will eat away at your profits!
I hope that this will help you in deciding if affiliate marketing is for you, Good Luck