First time painting decking, help

[Echofallen]

DH put a mat over the decking area on our garden room of our new house and it's loosened the waterproof paint work due to the rain so needs repainted as there's lots of exposed wood now. I've never done anything like this before. How do I prepare it? Will sanding it be ok or do I need to use some treatment to clean it and make it ready for a coat of paint? I have no idea and am getting confused. If I need to sand it is there a particular type of sandpaper I need? There was lots of different types at B&Q and I got confused and just left it.

I'm rubbish at DIY (as is DH) but need to get this done. Have the tub of paint ready in the hut from the previous owners. Just need to know what to do and I'll get on with it (today is dry so fingers crossed). Thanks for any advice!

Is it painted a colour or stained/ treated with oil?
If it isn't already painted, I wouldn't. Looks awful as it wears.
Do you have a power washer? That takes it off well but does depend if you are planning to redo it the same as it is now or change it

Hi it's painted already, one of the Cuprinol pastel shades, was going to use that. I have a new power washer but I haven't used it yet, it intimidates me a bit haha. I had started sanding off some of the flaking paint with a sheet of sandpaper but it needs something that will remove it faster.