Presumably a wooden door.
If you want it to remain wooden colour, I'd use Osmo exterior oil, gives a great tough finish and easy to put on.
If you want a colour, you'll need a primer (most double up as an undercoat and most are water based). Easy to put on, but take care you get good coverage. (Top tip, when using water based paint, wash your brush first, stops paint sticking and makes much easier to clean when done).
For the top coat I prefer an oil based paint, I find it much more durable. Trickier to get on cleanly and takes longer to dry but worth it.
Don't forget under the door (with the oil you can get coverage using a soaked rag).