When you choose a certain product - Packages you will see the option of availing credits in the lead form. The page will tell you what is the maximum number of credits you can use while making a booking. If you have accumulated enough credits you will have the option of using credits as one of the payment methods. For more info check out Tripotos guide to credits https://www.tripoto.com/collections/travel-for-free-with-tripoto
Just select a good trip which you are interested or any workshop.. you cannot use your whole credit for one and take the trip or workshop as free. .. you will have to pay for the trip and use few of the credits transfer.....