Outside of Morning Glory Orphanage is a beach that will always have certain items like a wooden crate that has many different kinds of items in it, smaller boxes that can give you items like gold plates and other materials for modding weapons or you'll get trash items such as sticks, bottle, etc near the water edge. You need to pick up 30 "trash" items meaning it has to be trash items like broken umbrellas or bottles that Kiryu will say he'll throw away. Any item that goes into your inventory is not considered trash.
You can keep checking the beach each time you come back to Morning Glory but there's a quicker method. Have at least 1 thing of fishing bait on you then go to the fishing rod on the beach and select "go fishing". Once in the fishing menu, immediately exit it, and a new wooden crate/small wooden box or trash item will spawn right away.
The box items on the beach have a small chance of containing trash (you'll usually get items like springs, textiles, or bait that doesn't count towards this achievement). So I'd recommend keep going in and out of the fishing menu until you don't see any boxes on the beach. This achievement may take a while to get if you happen to be super unlucky with spawns.