sounds like it is painted.
OK, then this is simple as.
remove the door furniture - knocker, letter box, number etc.
You need to sand it back aim for going back to the bar wood where you can and make sure it is flat if you leave any paint in places, Consider using very strong sandpaper (90 grit) or even renting a sander or paint stripper take care and read the instruction if you do rent something. If using a random orbit sander then you WILL need to finish by hand - go WITH The wood long strokes.
filler were you need two let it dry and then sand it back. Include all those pesky joints etc anywhere were there is a crack. Use proper wood filler and a pallet brush fill in stages and make sure the filler is flat with the wood when you are finished sanding and then primer let that dry
Then atleast two coats of UNDERCOAT first go up and down with the brush - then let it dry lightly sandpaper to knock back the high points you are NOT trying to remove the paint just make it look flat, followed by a quick rub down to remove dust (use A LITTLE spirit on the rag) then prime again this time going left to right, let it dry and then sand back lightly (240 ish grit) rub down again. in the same fashion as above
Then gloss - one coat let it dry (a good 24 hours if not longer) then sand back lightly again with 240 grit or even wire wool. Then time just aim to make the gloss look slightly DULLER than befor
Then final coat of gloss.
Remember to do all the jams - bits that face into the frame
pay extra attention to the top of the door and the bottom.
Take your time and work SLOWLY. use LONG brush strokes with the gloss or even a gloss roller on the larger pannels etc. then dont touch it untill its dry.
When your done replace the door funiture or by new ones if you like
FYI : If you want to change the colour than make sure you use the right undercoat, light grey for light blue, yellow, etc. DARK grey for Dark green, Dark Red etc. A good paint shop etc will help you out.
Use good quality paint = crown trade paints are very good if you have one locally.
I tend to do, 2 primer, 3 undercoat and 2 or 3 gloss but then I like my doors to look pristine and the paint work lasts for ages.
Remember 80% of your time should be on sanding, filling, priming and undercoat then 20% of your time should be on gloss etc. This will give you a great finish get the bones right first.
HTH