Answer: Sometimes it's easier and more cost effective to completely redo a lawn than to try to salvage it when it's really in distress. I'd rent a rototiller, till then entire area, rake out all the debris, level the ground and broadcast fresh seed. Grass seed will germinate in 7-10 days and in less than 6 weeks you'll have a brand new lawn.
If you really want to try and renovate your existing lawn, start with killing the weeds, pulling out the tree seedlings, raking out the moss and dead grass, then renting an aerator (to improve the compaction of the soil). After these steps, reseed and water well.
Best wishes with your lawn!
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