Washington accepted his nomination as commander of the Continental Army with this quote, "I beg it may be remembered that I, this day, declare with the utmost sincerity, I do not think myself equal to the command I am honored with."
While he might have been trying to modest, it was also certainly true. He was almost definitely the most qualified American for the job, but that didn't mean he was truly qualified to lead an army. Washington, in 1775, had at most commanded a regiment. He'd never led an army of any type. Washington definitely shows he is the man for this job, but he is learning on the job. I'm currently reading His Excellency by Joseph Ellis (one day when I finish it, I'll post a review, but so far I have to admit I'm loving it) and this is a point Ellis stresses. Ellis also stresses the huge challenges that Washington faced with the Continental Army, which was never a true army in the way we think of today. I have to admit that realizing this and really looking at the challenges makes Washington's successes all the more incredible.
I found this page on the beginning of Washington's leadership of the Continental Army:
Any military expert would have given the Continentals little chance. After all, King George's army was the best-trained, best-equipped fighting force in the Western world. The matchless Royal Navy could deliver an army to any shore and strangle enemy nations by blockade. England's forces were commanded by career soldiers who were veterans of wars all over the globe. In sharp contrast, the colonial force staring them down was less of an army than a large gang. Its soldiers came and went almost at will. The officers leading them had little command, let alone fighting experience. Furthermore, in the colonies, support for the rebellion was far from firm.
Washington's first duty was to turn this unruly crowd into a real army by instituting disciplinary regulations. To facilitate his efforts, he urged the Continental Congress to provide enough money to pay for longer enlistments for his soldiers. But when New Year's Day dawned in 1776, much of his army had gone home because their enlistments had ended.
Washington first commanded American forces arrayed around Boston. Using cannon captured by Henry Knox from Fort Ticonderoga and heroically transported miles to Boston, Washington fortified a high point overlooking the city. Unnerved by the colonials' sudden tactical advantage, the British withdrew from Boston by sea. Washington, however, had no illusions that his enemy was finished. The question was where they would strike next.
By spring, it was plain that the British plan was to seize New York. It offered several advantages including a large port, the propaganda value of holding one of the rebels' biggest cities, and a route by which troops could be delivered to the American interior via the Hudson River. Washington moved to stop them. In July—a few days after the Declaration of Independence was signed—the British landed a huge force on Staten Island. By August, 30,000 troops marched on Washington's force. On their first engagement late that month, much of the Continental army either surrendered or turned and fled in terror. On September 15, the British landed on Manhattan, and again Washington's troops ran away. Enraged, he shouted at them, "Are these the men with whom I am to defend America?" A day later, his troops were resolute in their defiance and won a small engagement in Harlem Heights. But by November, the British had captured two forts that the Continentals had hoped would secure the Hudson River. Washington was forced to withdraw into New Jersey and then Pennsylvania. The British thought this signaled the end of the conflict and dug in for the winter, not bothering to chase the Americans.
Washington now realized that by trying to fight open-field, firing-line battles with the British, he was playing to their strengths. He turned to tactics he had seen Native Americans use to great effect in the French and Indian War. On Christmas Day, he led his army through a ferocious blizzard, crossed the Delaware River into New Jersey, and surprised an enemy force at Trenton. A few days later, he took a British garrison at nearby Princeton. These actions were less large-scale battles than they were guerrilla raids. Nonetheless, these minor victories gave his army confidence, brightened the spirits of the American people, and told the British that they were in for a long and bitter struggle.