To unlock this trophy you need to find 30 attachments, and since you start the game with roughly 30 you will need a total of 60 - 62. You can view how many attachments you currently have in the objectives board menu by going to the attachments tab towards the top and opening it. Once opened you'll see how many you have out of the total (89), as well as which ones you own.

In this menu if any attachment is blue, that means you currently do not own it, however you do know the location of it on the map. These blue attachments can be pinned and tracked by pressing  over them, and you can find more attachment locations by clicking the related option when collecting information in the world, should that option be present.

There is a much quicker way to earn attachments than hunting them down however, you can buy a good number of them from the store with credits. These attachments are somewhat costly, so you may want to find some and buy what you can. Upon having a total of 60-62 attachments open the loadout menu and hover over one of your weapons pressing  to edit it in the gunsmith. On the right side you'll find what attachments are currently equipped and by pressing  over any of them you can change the attachment out for another. Do this having the required amount and the trophy will unlock.