Hanphul, Acadia's post looks daunting, but it's extremely useful information.

How to fit an item to a body in Poser depends on whether the item is a Figure, or a Prop, or a Hair Prop (I think Hair Props follow the same logic as all the other Props, until somebody contradicts me).   A Prop, such as a hair-prop aimed for Aiko3, should head for the Aiko3 character that is selected.   Obviously, selecting the right figure and bodypart is essential.

With a hair or clothing figure, the intended character doesn't actually need to be selected.   The new hair or clothing figure will appear at  x=0, y-0, z=0, anyway.   Under the "File" pull-down, there is a "Conform to" selection.   Pick the intended target.   You may need to modify the clothing/hair a fair amount, to get a good fit.

Now, if you are already doing all of this and your problem is clumps of hair protruding through shoulders, or through items of clothing, please provide details and screencaptures.   There are lots of people here who will be happy to help.   For some reason, the crucial part of a Hair figure is the Neck (I've never worked that one out - can someone explain it?).    This is usually where the morph dials are to manoeuvre the hair strands clear of the face and clothes.