If you want to do quests, you need to click on a game (that you want to do the quests for) and then, there will be an orange button that says register, click that. From there you will be given a badge to indicate that you have registered for it and after that, you can do the quests!
Hope it helped!
On Roblox it literally takes thirty seconds to create an account.
On Bananatic, it will take up to a day to realise you have created an account. Make sure you have created a new account and that you are not logged into other accounts on the same browser. Some browser plugins may block this feature. Also make sure you have used the link on Bananatic and not just created an account.