How can I display posts close to the users distance?

Hello there,
first of all I am brand new to web design. I want to let users add posts that are linked to a location. For the location they enter an address (And I receive the coordinates as described in this video https://www.youtube.com/watch?v=6Pd-zuRWKhQ ). I do not know how to start and store the coordinates in the database as all I can do is use the graphical dataset widget to store the user input.

Also later on I need to query the entries by distance to the user but i assume some math in the query algorithm will do.

Please help me I really could not find anything on the net understandable to me.