Should I charge by the sq. ft.
What equipment should I buy?
Is it a smart idea to hire anybody starting?
Is it worth doing residential cleaning?

Well it woiuld be nice if you told us more what type of business, I will assume cleaning?? What type of cleaning, window washing ? Side walk cleaning,

But to answer this in general,
1.yes you should, but sometimes you can't and have to estimate time
2.what equipment you need to be in business, or lease larger pieces
3.only if you have enough jobs and contracts to afford them. Also do you want an employee or independent contractors, ** liablity insurance on them, workers comp insurance, in some states requried health insurance and so on.

Business and work is worth it if you can make a profit doing it.