Any advice?

1) Get familiar with your building codes right now. They are there to keep
people from making major mistakes.
2) This is not a DIY project. It is a DIO (ourselves) project. A two man crew
is marginal, three is not bad, four will get the job done much better.

How do I ensure the building is square?

By knowing what you're doing (think equal diagonals).
Your last question indicates limited knowledge of building methods. If
financing a garage of this size is part of your plan, the bank may well be
happier if you went with a supplier such as Morton Buildings or similar. You
could have major problems finding materials that match their quality standards
in the box stores. HTH

Joe