I bought a mac mini and a Dell P2415Q Monitor. What cable do I need

The monitor has:

Connectivity

DisplayPort in

Mini DisplayPort

DisplayPort out

HDMI (MHL)

Speaker-line out

4 x USB 3.0 downstream

1 x USB upstream

And comes with

Monitor and stand

Power Cables (varies with countries)

DP-mDP cable

USB Upstream cable

Just use the included DP-mDP cable and plug the nDP (mini displayport) end into the Mini's Thunderbolt port and the other end into the monitor.

Note that only the 2014 Mini will get 4K and then only at 30 Hz

What cables you use depends on the inputs the display has. You will need a Mini-Display Port to whatever connection you want to use. However if the display has a HDMI input you can connect the MM's HDMI output to the displays HDMI input.

Why didn't you just buy a 5K iMac instead?