Guacamole can be a little tricky to store. It can easily oxidize but there are a few options. If you're serving only a certain amount, put it out right away and place the rest in a container and cover the guacamole with plastic wrap or wax wrap. Make sure the wrap makes contact with the guacamole and try to have as little air as possible between the guacamole and the wrap. Then cover with a lid. If the next day there is some oxidation, you can just stir with a spoon or remove the brown part on top and give it a stir.