Lemongrass looks like a long stick, somewhat like spring onion. The name itself says everything about it, that is that its fresh aftertaste reminds one of lemon. Typical of Chinese cuisine, but also of Thai, Vietnam and Indian cuisine, it is usually coupled with ginger. Very many varieties exist that grow in the several countries. It can be bought fresh, dried or powdered, usually in the best supermarkets or in ethnic shops.
Once you bought it fresh, remove the outer, tougher leaves (something like what you do for spring onions or leeks) until you reach the softer, innermost part. Cut off the final part where the roots are and now decide how you will cut it: if you like its intense taste, you can cut it into thin discs (starting from the final part, like for spring onions); if you like its taste, but you don’t want it to prevail in your mouth, cut it into little blocks or stripes that you can put aside when you find them in your plate. The initial part where the leaves are should be thrown away.
Dry lemongrass usually comes in sticks: add them starting with half a teaspoon until you reach your desired taste. If you buy the powdered one, begin with the tip of a teaspoon, because the taste is very concentrated.