What were the main reasons for the French and Indian War?

In the middle of the 18th century (1700's), the British and the French were both trying to establish positions in the Ohio valley, including western Pennsylvania. Bear in mind that the British and French had been enemies for centuries in Europe, and had fought several wars.

Before long, British and French forces clashed over their attempts to gain control of this territory by establishing forts in the area.  Indian tribes who were unhappy with the British efforts to expand westward joined with the French.  There were minor conflicts for several years in the Ohio valley as well as further north into the Great Lakes and Canada before the war was officially declared in 1756.  

The turning point of the war was in 1759 when the French surrendered their garrison in the city of Quebec.  By the end of 1760, the British had gained control of the whole area.

This was confirmed with the Treaty of Paris in 1763, which gave all of North America east of the Mississippi River -- other than New Orleans -- to the British.