+1 to all the above.
The boil moves the wort around and that also helps it from sticking to the bottom. When adding your extract, you really should turn off your heating source and stir until all of it is dissolved before turning it back on. Again keep stirring until you're sure it is all dissolved, and "moving."
If you see a brownish sticky spot on the bottom of your kettle, when cleaning, that's an indication of scorching. During the boil, scrape the bottom with your stirring spoon or paddle so nothing gets a chance to stick. Once something sticks it accumulates faster.