Start with 3 bags or boxes - 1 for stuff you want to keep, 1 for stuff you want to donate and 1 for stuff want to throw away, you could have another for recycling.
Find the area you want to declutter, start small - don't tackle more than you have time to finish. Put some music on and get started.
Start by throwing away all rubbish - this is the easy bit, it will clear space so you can see everything else.
Put away anything which has been left out.
Put anything which doesn't belong in here into a bag or box, ready to be put back where it belongs.
Put anything you don't want to keep in your donate bag - be ruthless, if you haven't used it in the past 3 months get rid of it. If it doesn't have a home get rid of it.
Vacuum and dust your room - it should now be tidy. There is just that bag of stuff to put away.
Sort through the bag - do it now, not later. Be ruthless - if it doesn't have a home find one or get rid of it. Ask yourself 'do I really need this?'
For example, I have 2 of these. Do I really need 2? Neither are being used so I'll keep one as a spare and get rid of the other.
Enjoy the results. Beware you may find yourself addicted to dejunking.