Over the years many schemes have been developed to evaluate the strength of your hand. But the one that has endured and is almost universally used today is High Card Points (HCP) sometimes refered to the Milton Work Count. It has the advantage of simplicity, and is quite accurate, especially for No-Trumps.
Using this scheme there are 40 points in total in a deck of cards, and the average hand will hold 10 points (one Ace, one King, one Queen, and one Jack).
Most people add up their HCP and then mentally make adjustments to the count. Aces and Kings are slightly undervalued by the 4-3-2-1 scale; Queens and Jacks overvalued. Tens are also useful especially in No-Trumps when accompanied by other high cards. Generally it is better to have your high cards in your longer suits. You can find more information from these links NZ Bridge Lesson on HCP, Hand Evaluation on Wikipedia, Hand Evaluation - not all points are equal.
You can practise counting your high card points below...