spell

How do you spell your name?

To "spell" a word is to say the letters in the correct order.  Notice the following:

1. Can you spell the name of the company?
   
2. I'm not very good at spelling.
   

past

The past four months.

The "past four months" is the most recent four months, starting now and going into the past.  Notice the following:

1. In the past I worked in a post office.
   
2. He has lived in Ireland for the past four years.
   

What do you do?

What do you do in Japan?

"What do you do?" is like asking "What is your job?"  Notice the following:

1. What does she do now?
   
2. What did you do for your first job?
   

that's it

I know how to program and that's it.

"That's it" is another way to say "that's all" or "nothing more." In the example, the speaker is only good at programming computers, not fixing them.   Notice the following:

1. Just push the button to start the machine and that's it.
   
2. I ate apple and some crackers and that's it.
   

good at computers

Were you always good at computers?

If you are "good at something" you have excellent abilities in that area.  In this example, the person is good at programming computers.  Notice the following:

1. He was very good with numbers when he was a child.
   
2. She is good at public speaking.