if elon musk colonizes mars the first could he make it his country, and if too can i become a citizen?

A country can’t claim it but an individual can 

No, there is an agreement between all countries with the capabilities of traveling in space that no one can lay claims to another body in space, they are there for all. Otherwise the United States could have claimed ownership of the Moon.

China is going there for sure

"Colonize" and "colonizers" are politically incorrect terms now. If you use those words you'll be labeled a racist. Save yourself!

the one way  ticket and starter condo will be several billion dollars .

You'd have to meet the eligibility requirements. Mars won't be a place that can tolerate economic parasites. There won't be any "welfare," and probably very little voluntary charity, either. You pull your weight or you die. You abstain from making trouble for your fellow colonists, or you die. You intelligently deal with the rigors of the environment, or you die. When the environment is anywhere away from Earth, only the people who are smart, resourceful, and well-behaved will survive.

He's not going to colonize Mars. Humans couldn't survive on the planet and we don't have the technology to mimic the Earth's environment.

There is nothing to colonise there. With present-day technology we can send a few people on a one-way trip, but they will die once the food or heating runs out.

No food grows there, no wood to burn, no water to drink, no breathable athmosphere, no magnetic field to keep out/slow-down charged particles...

No.  The Outer Space Treaty rules that out.

There is an international treaty that no person or country can own a planet or moon or any part of it. But I'm sure that will change will people start living on the moon or Mars. So maybe there will be a country called Muskville with him as king.