First of all: a lot of us have all been there. Many of us have so many things that we don't even know where to begin when we decide to try to clean up! Give these steps a try and surely you'll have less clutter in your life!
Throw away all the trash!
Firstly, throw away any old food or food wrappers/containers. Recycle as much as you can, and if you're not sure what can be recycled, there are tons of websites that can help! Grab some trash bags and fill them up!
Get rid of/Donate things you haven't used in 3+ years.
Odds are if you haven't used or worn anything in three or so years, it's time to let it go. Books, clothes, shoes, pens/pencils/markers, games. Donate them to people who may have better use for those items.
If you're unsure: put things in piles.
One pile to reconsider where it'll go, the second for getting rid of/things that don't make you happy, and another for donation.