Do you try to remember everything you have to do? You're bound to forget a task or two and even if you do remember them all, you have to keep it in the back of your mind until it's completed.
It's more efficient to write down all your tasks. The process of writing it down allows your mind to let go of the task and focus on what you're working on. Keeping the list online lets you get to it from anywhere.
What online todo list should you use? There are a ton of options, and I've included all the ones I know of. (think of all the DP you'll earn!)