Question

How did the american revolution contribute to the start of the French Revolution

Answer 1

A huge part of it had to deal with the way that the French government dealt with debt. 
Only their third estate, primarily poor merchants and peasants, paid any taxes. The French largely funded the American Revolution because of their long-standing animosity with Great Britain. However, the debt they incurred was only slowly paid off because the people they were taxing had very little money. The country fell into an economic crisis and resentment began to build against the first and second estates--the weathly, title-holder, landed gentry and clergy. Thus the revolution. 
A good comparison is Great Britain post-America Revolution, who taxed their citizens more fairly and avoided revolution by not throwing most of their citizens into the desperate straits of poverty.

Answer 2

"it provided french citizens with a successful model for a form of government" on apex :)