The Torah (Bamidbar 31:23) commands us that utensils made of six metals which were acquired from a Gentile must be toiveled (immersed in a mikvah) before they may be used with food. The six metals are gold, silver, copper, iron, tin, and lead. Glass utensils must be toiveled as well, based on a rabbinic requirement. If one purchased used utensils, they must first be kashered before the tevilah. However, if one borrows or rents utensils from a Gentile, there is no mitzvah of tevilas keilim. Before immersing, the utensils must be completely clean. All labels and even residual glue from the labels must be removed prior to tevilah. Prior to tevilah, a beracha is recited.
Most regular items can be immersed without ruining them. For certain electronic items the water should be shaken out, and one can use a blow dryer to dry the inside or it should be left to dry for three days. If it has a digital panel, and one is scared that it will get ruined, he should gift it to a non-Jew who will lift it up and take possession and then borrow it back from him.