What I do is i select the area i want to blur using the lasso tool. I select all of the background except for the subject I want to keep in focus.
Now select the background. Click on Filter > Blur Gallery > Field Blur and use the dial to however much blur you want. You should move the pin to the areas you want blurring
There is also gaussian blur which, when applied, allows you to use the paintbrush tool with the layer selected and allows you to unblur certain parts, depending which primary colour you chose.
You can use the blur tool but it isn't very good. You can also do this other ways. look around