You need to mask out the person you wish to remain sharp, by 'selecting' them. There are various ways to do this. The most precise way is the Polyganol Lasso tool - you simply click a line around the subject.

There are also automated ways of doing it, the success of which depend on how complicated the selection is: if the subject is isolated on a neutral background, the results are generally good. If the background is 'busy', you might have problems.

Check your Photoshop help file for 'Making Selections' or Google the same for heaps of tutorials.

Once you have made your selection, you need to 'invert' it, so that you have selected everything but your subject. You should then Feather the selection, which softens its edge and prevents an artificial looking transition between the subject and background - about 5px is generally fine.

Finally, the most realistic blurring option (i.e. the one that looks most like depth of field blur) is the Lens Blur filter (Filter Menu > Blur > Lens Blur). You can tweak various options to get the result you want.

This is just an overview of what you need to do: there are hundreds of tutorials online. This answer should help you find the right ones.