Decide how much clearing out you're going to do, it's best to do in sections otherwise you'll get confused and frustrated. Plus it's much easier to feel good afterwards even though you've only done half a bag ;)
Create three bags called Charity (to donate to charity obviously), Sell (items you wish to sell on) and Bin (items beyond donating or selling). Make sure these are clearly labelled because it's a real bugger when you accidently put an item in the wrong bag and have to empty it, it stresses me out just thinking about it. If you can then use different coloured bags and put them in different corners of the room.
Choose your rail/box/wardrobe and start small.
Try on everything and mix with other outfits to see the potential. You can chuck anything you hate into one of the piles, in fact why didn't you do it sooner? Even try on things you like and be honest with yourself, do you actually wear it and does it suit the rest of your wardrobe? How long has it being since you wore it? Is it damaged? And if so would you repair it? If you're unsure whether you would want to wear it again then let it go, you'll probably find something similar in future.
If you want to sell or donate something then check the condition, there's standards and to be honest it could have being ages since you last looked at it. Turn it inside out and hold it up to the light, you're looking for flaws such as fading; stains; bobbling and consider what condition you would want an item to be in when you buy it.
Fold away any items into the bags, it allows more room. Also pop a scented soap bar in to prevent moths and give your clothes a pleasant scent.
Now you need to organise what's left, pull out everything and sort into clothing type. Start with one type and colour code it if you can (although mines more shades of black and grey) then move onto the next type, if you can then tie a ribbon inbetween the sections or use different coloured hangers. If you have a big enough wardrobe you can sort it into casual or smart and the season, lucky you! Although if it's Summer I recommend storing your Winter coats where moths can't nom them.
Consider how to organise your accessories, there's plenty of ways to store and display them. I have a antique coat stands purely for hats and use a jewellery tree to hang my necklaces off. Think outside the box! I use a cute pillow to display my brooches and use old jewellery boxes to create sections in my drawers, it's annoying when I have to hunt through it to find a tiny ring.
Hanging organisers are perfect for storing your shoes without taking up much room or you could nail a ribbon to your wall and tie your shoes up, I only do this for my shoes with heels (otherwise there's nothing to tie onto!) and make sure they're clean.
Now it's all clean and organised you can now consider decorating but that's for another day, now you can give yourself a big pat on the back/hot chocolate/new pair of boots for hard work.