How to make a reminder application in C#

The first thing you have to do is realise that your reminders won't display unless your app is running: so you need to decide if your app should be running all the time, or kicked off at regular intervals by the operation system to check and display if it needs to. The two approaches are very different, and need preety different code.

Other than that, it's pretty simple: store a list of reminders in your database (so they persist once the computer is switched off) and read them back when your app starts.
If your app is running occasionally, get the time, check your list, display any "ready to go" reminders and remove them from the DB.
If you app runs all the time, set up a Timer, and handle it's Tick event. In the Tick handler, do the same as you did for an "occasional" app.

So start by writing code to access your DB, compare lists, and then display messages.
Then expand from that in one direction or the other.