Using Ardunio and MOSFET to drive 5V to 12V

Firstly images can be included inline using the Insert Image button....

You have the load on the source side of the MOSFET - no good, it must be on the drain side.
However that doesnt account for Vgs never reaching 0V, and I can't figure out what the second
connection to the base is - a 1k to ground would be sensible if using PWM to speed up switching,
otherwise nothing is needed there.

Check you've got the NPN pinout correct?