A Nmosfet needs to have a higher voltage to the gate compared to the source so it is usually used as a lower side switch to provide ground to the load, the source will be connected to the ground and the drain will be the output.
It can be also used as a high side switch, in that case the drain is connected to the positive supply and the source to the load but in that case you will have to use a method like bootstrapping or an isolated power supply to actually drive the gate with a voltage about 5-10v higher than the drain voltage.
The problem is that when the mosfet conducts then the source voltage will be almost as high as the drain voltage so you need to provide the gate with a bias higher than that.
The Nmosfet has lower Rds-on (on resistance), is cheaper and also has lower gate capacitance, that is why is is preferred for higher efficiency (like for example switching PSU).
A Pmosfet is the exact opposite of a Nmosfet, it works with a voltage in the gate lower then the one connected to the source, it is usually connected as a high side switch, source to positive supply and drain as output.
You probably intend to drive the mosfet from the mcu with 3 or 5v, if what you want is a switch then a Nmosfet would be the obvious solution connected at the ground side of the load.
You also have to select a mosfet that has logic level gate threshold voltage (turns on at a low gate voltage).
Alex