Beginners Help Required- Painting Woodwork / Skirting Boards

Hi there, Just starting to decorate my 8 year old house. The door frames, door, windows and MDF skirting boards are all painted white. It is certainly not emulsion but I am not sure whether it is satinwood or gloss? Anyway to tell? It is a Barratt house by the way, 8 years old.
I am looking at rubbing these down lightly with sandpaper. Should I be using normal sandpaper or wet n dry sandpaper and do I need to wet the wetn dry? Also what sort/grade? Coarse, medium or fine(what grade). Or should I just go through all to get a good finish. I was going then to undercoat, then rub down slightly again (Sand? WetnDy? What sort of grade?) and then another coat of undercoat then rub down again.
Then either using gloss or satinwood as a final coat (Depending on discovering what I have already got?). I was gong to apply a coat and then rub down before the final coat. What type and grade of sandpaper or wet n dry would I use for this. Sorry for all the questions but this is the firat decorating for me, so I am new to all this. Thanks ever so much for your help. I just want to make sure I do a good job andthen not have to do it again for a good few years!

you only need to lightly rough up the woodwork so as to get the new paint to adhere i:e to make it key,medium grade glass paper i think would suffice,
if there is any bare wood then you need to prime 1st,then undercoat then top coat(gloss/etc?)you could then lightly sand down again if there are any high areas (ater the undercoat),i:e fibre starts to lift through painting,but i think that is unlikey as it is already painted.
no doubt someone with a lot more knowledge of decorating then myself will offer some better advice.
hth a bit.

Just give the woodwork a light sand with aluminium-oxide paper to key it , probably 100 grade .
Apply undercoat first if using glosss .
If using eggshell or satinwood then apply 2 coats & skip the undercoat as theyre self-undercoating.
Stick to oil-based paint as the quick-drying/acrylic stuff is fiddly & not very hard wearing