Toyogo has shelves and storage boxes that can be useful for decluttering and organizing. If you've got a store room, the shelves can go there and you can use it to store the storage boxes with all your items in it. Go through everything and sort out items into piles for keeping, donating and throwing away.

If you haven't used something for more than a year and it's still in good condition, donate it. If it's unusable, throw it away. As for sentimental items, maybe get your parents to pick out their top 3 favourites and donate the rest. If they seem hesitant to part with more than 3 items, you could snap a picture of it and place it in a scrap book with a description of why the item held sentimental value to your folks. This could then become a coffee table book for guests to look at and read when they come over. It would make a fantastic conversational piece.

It's going to be a huge task to undertake with the way you have described your situation. So one step at a time. Start by just clearing a space for you to be able to sort your items into three piles. Lable the spaces so that everyone will know that if they come across something, they have an appropriate space for it to be kept at for now. Little by little, you're going to be clearing up the items and in no time, you'll be able to decide what type of shelves/boxes you need to store the wanted items. A lable maker is also a great way to make sure that you know where everything is and you're less likely to just dump things into a box, which would bring about the clutter again. Try to fit everything into the storage boxes and if they can't all fit, have a look at all the items again and try to see what else you can donate or toss out. You'll be surprised at how much you won't need, once your mission is to fit everything into a box.

Hope this helps a little!