go to a doctor

When you have small health problems like colds, flus, and infections, you usually go to a doctor's office or a clinic. You can call this "going to a doctor" or "going to the doctor".

This is different from "going to a hospital". A hospital is somewhere that you go when you have major health issues that require you to stay several days. You'd go to a hospital if you're getting surgery, if you had a heart attack, or if you're giving birth.

for God’s sake

We use use this for very strong emphasis in personal matters, especially when asking someone to do something. It conveys a feeling of frustration and anger.

Turn the radio down, for God’s sake!

For God’s sake! You’re not even dressed yet!?

Notice that “for God’s sake” can be at the beginning, end, or sometimes the middle of the sentence.

Even though the word “God” is used, there’s often nothing religious about this phrase. Many people use it without thinking about the idea of God at all.

get (something) checked out

If you have a problem with your body, car, house, or anything else, you need a professional to “check it out.” This means to examine it, and find out what the problem is and how to fix it.

You should get that lump checked out.

Your friend points out that your car is making a weird noise. You say:

Yeah, I’ve been meaning to get that checked out.

You can also say "get (someone) to check (something) out":

You should get a doctor to check that out.

But we often use “get (something) checked out" instead because it's usually obvious who (a doctor, a car mechanic, etc.) will "check it out".