“You have to keep plugging away, you know?”

A friend of yours is an actor. He auditioned to get a part, but he didn't get it. Now you're trying to cheer him up with some friendly advice. You say:

You have to keep plugging away, you know?

You have to (do something)

Use "you have to ___" when you're giving someone advice, especially when you're an expert or know more than the other person. For example:

The bottom line is that you have to reduce your calorie intake to lose weight.

plug away at (something)

To "plug away at" something means to keep working hard on it for a long time, without expecting a quick reward.

The example above uses the shorter version, "keep plugging away". The full version uses "at":

Keep plugging away at your acting career, and I'm sure that one day you'll be a huge star.

Although it describes working really hard, the phrase "plug away" sounds positive and friendly.

you know?

Some English speakers, especially younger ones, use "you know?" on the end of their sentences. There's not a lot of meaning to it, but people use it when they want the listener to agree with the feeling or emotion of what they're saying. For example:

I kind of feel obligated to go, you know?

In this example, someone doesn't want to go to a party, but she feels like she has to. She wants the listener to agree with that feeling, so she says "you know?" at the end of the sentence. Another example:

You can't beat face-to-face communication, you know?

There are some people who over-use "you know" and put it in almost half of their sentences. It doesn't sound very intelligent when you do this.

When someone who's speaking to you says "you know?", it's good to nod your head or say something like "Yeah."