This is the first but definitely not the last time I'll say something like this: you probably don't need another productivity tool. Planners, apps, reminders, lists, sticky notes, etc. they all can help. But only one thing determines if they will work- consistency.
Don't get me wrong, I LOVE sharing tools and tricks and apps and things. After talking with countless friends about their busy lives and schedules I've found that it's not that they lack some system to organize themselves, but that they have way too many. Trying to use every system all at once is worse than only using one, even if it's not dynamic enough to manage every single detail.
So the TL;DR:
Pick a Task Management System and Stick To It.
We have either been or known the person who has a planner but can never make heads or tails out of their life because its never around when they needed it. What good is a planner that is missing details? And how can you make a game plan for your day or week if you don't know what needs to get done? The simplest part of tracking your tasks is the hardest because it requires you to form a habit.
If you're hoping to get started, try carrying a pocket sized notebook for a week. You'll find that even though it's not the most tricked-out fancy app, it's a good start. Simply alleviating the mental burden of the things swimming in your head is a step in the right direction.
Here are a few posts where I share ideas and best practices for task management: