You can use the phrase "go in" to talk about approaching something. For example,

Sometimes when you go in to pet her she'll, like, freak out and try to scratch you.

People usually use "go in" in the phrases "go in to ___" or "go in for ___". These phrases explain the reason for approaching. Use "go in for ___" with a noun:

I tried to go in for a closer look, but as I approached it swam away.

Use "go in to ___" with a verb:

The other guy went in to try to punch him, but he grabbed him by the arm and slammed him on the ground.