Some of the residue looks to be mold so the recommendation would be to contact a professional to have it safely removed. After the space is cleared of any mold, a high-pressure wash and some detail scrub brushing will likely take care of the rest of the dirt and grime.

For organization, there are some affordable, sturdy metal shelving units with adjustable shelves at retailers such as Costco and various hardware stores.

Happy cleaning!

Pressure washing & an organization expert help appear to be needed.

Are there water leaks that are supporting mold growth? Is part of it underground?

Cheap shelves. You can get them from Ikea or other places. Ikea lets you design online.

You could also install a thick board across the top of the side wall and use hooks to hang up things like ladders.

A cleaning would go along way.

To continue on Bri's comment:

. . . Put every thing you didn't throw away under a tarp outside. As you use things under the tarp put it away (washed) inside the garage. after two months, throw away whatever is left under the tarp.

Uh, give it away or donate it if it's usable. The landfill will be the downfall of humanity.

Free standing garage not attached to the house?

First, get rid of everything you don't actually want/need/use. Set it on the curb to be hauled off by either the local government agency that does that sort of thin and/or anyone else that wants the stuff.

Cover what you do want to keep with a tarp on your deck or porch or high ground so it's not setting in a puddle when it rains.

You will need to address the cause of the moisture problem before any cleanup will be worthwhile. So at least figure out what the problem is before investing heavily in keeping a free standing garage or a garage added to a home that is fairly easily removed.

Does the garage roof leak anywhere -- even a little bit; if so, you'll need to repair or replace the garage roof -- and I'd suggest you add "attic storage" above the garage as you do so.

and/or

It looks as if your driveway may actually slopes downward toward your garage directing rainwater into it. If that is the case, cleaning out the mold and mildew would only be a temporary fix -- you need to prevent it from reoccurring.

If you want to fix this moisture problem as a DIY project then, my (not a pro) thoughts:

to keep the rain water out you may need to raise the floor level of the garage -- add more concrete so the sloped height difference is sufficient to turn the water --

and/or otherwise redirect the rainwater by cutting a slice out of the concrete driveway just in front of and parallel to the garage opening ... then

either create a concrete channel covered with metal drain grate to turn the water or. less expensive, create a deep gravel filled ditch that catches the rainwater and leads to a drainage path beside the garage.

If any of the concrete block wall has dirt against it outside, you're probably going to need to do some waterproofing.

Only after you identify the cause will cleanup actually be more worthwhile than just tearing the building down.

You should remove all the old paint from the walls as well as any mold or mildew. That might require sandblasting -- there are companies that do this, hopefully one near you.

@Mark Bischak

Are they supposed to throw away their snowblower, skis and sleds because they haven't used them in 2 months?

Make seasonal adjustments according to geographical location. Thank you Sigrid for pointing that out, I hate to see anyone lose out on the joys of snow-blowing. (I've had my share)

Freeing up floor space is the number one best de-cluttering agent.

these photos should be placed in the 'do you need a garage?' poll thread.