Home row basics
The home row is the middle row of letters. Rest your left fingers on A, S, D and F, and your right fingers on J, K, L and the semicolon key. Your thumbs rest on the space bar. Feel the small bumps on F and J: they help you find your place without looking.
From the home row, each finger reaches the keys above and below it, then returns. Always come back to the home row. This builds muscle memory.
How to practise
Spend five minutes a day typing simple words that use only the home row, such as "dad", "flask" and "salad". Look at the screen, not your hands.
See also: How to type faster, Accuracy before speed.