STEP 1: Try it on!
Deep breathe...this is the hardest part.
1) Try everything on in your closet! Just do it.
2) Now make three piles:
Donate or Consign Pile: Everything that doesn't fit, you don't like, doesn't fit your life, you haven't worn in 2+ years
Store Pile: Items you are keeping for sentimental reasons only
Mend Pile: Items you want to mend (are worth the time/cash to fix)
3) IMMEDIATELY get the piles into bags/boxes and get them out of your "working closet". Store the sentimentals in another area and put the bags of donate/consign items and mend items directly in your car so you can deal with them ASAP and they are out of your closet!