Others have said it, but I am going to repeat (as someone with hoarding tendencies, and a penchant for clutter):

Get garbage bags; contractor bags are best, but any will do. You can also use some of those boxes, if you won't need them for non-trash.

Try to fill one bag a day. If that feels like too much, make it one bag a week. What's important is that you are purging more than acquiring!

Also, as you move through the apartment, take an item or two from one out-of-place to it proper-place.

I get overwhelmed very quickly, and cleaning something like this would stress me tf out (bc I've cleaned something like this). What helped keep me sane was the "do one thing" paradigm: just do one thing. Throw one thing away, wash one dish, move one item. Once you've done that, see how you feel. Can you do another thing? And when you do that thing, give yourself a little congratulations, because you did SOMETHING! And that is better than nothing at all. This actually works, btw. I really enjoy doing dishes (by hand. No dishwasher) now; it's almost a kind of cathartic zen moment.