I want to replace my white bg through a transparent bg
How can i do this?

When you're making a new document there should be a dropdown box named "Background Contents".

There are three selections, white, background color, and transparent. Choose transparent, of course.

i want to replace the bg on a graphic with a white bg

Oh, say like a graphic that was off google and it has a white background?

Use the magic wand tool, increase or decrease the tolerance to your liking.

You can also use any of the lasso tools to render/cut out the image you want.

worst idea ever.. depends on what kind of pic it is.. but use ur pen tool