My espresso tastes sour, bitter, or unpleasant.

Based on the tastes you are noting, you can learn a lot about your extraction and adjust your variables (grind size, dose, water temperature, etc.) to achieve a smooth, well-balanced shot. 

Typically, a sour taste is indicative of an under-extracted shot. Lengthen your extraction by lowering the dose to get more liquid to the coffee, but keep your extraction times up. 

bitter taste is usually indicative of the opposite - an over-extracted shot. Experiment with increasing your dose but grinding coarser, and shortening your extraction time. 

Additionally, keep in mind that the roast itself may be impacting your flavors. Lighter roasts are typically less soluble and therefore brewed with lower temperatures will yield sour and salty flavors, darker roasts are highly soluble and therefore brewed with higher temperatures may taste burned or bitter.

If after experimenting based on the above, the taste is still not hitting the mark, consider that the roast itself may not be your taste. We love visiting our local shop to test a house roast as an espresso as a benchmark, and if we like the taste, buy a bag of beans to mimic at home! That way, you'll know you like those beans as an espresso.