How to make people know about my project?
It's just a simple task manager - http://todox.app
I know there are a lot of such apps, but its completely free and quite simple. I think some kind of audience may find it useful. Please advise how to promote such things. Thank you!
Try targeting a specific niche. A niche market so small that big players like Trello wouldn't waste their time to care. Now when you find your niche, understand what special features they need on top of your "typical to do app" to attract only this group of people.
"I know there are a lot of such apps" < you've answered that yourself :)
There's not a lot that sets this apart from other todo applications, Take a look at Trello, it's got a similar feel, more features, and a mobile app.
You should charge for it.
Seriously?:) How can it help make people interested?