Of course, there are rules how nouns build their plural in German but it's not so simple as in Portuguese or English. There are a lot of rules and it will be quite difficult to learn them for a short period of time. The same situation with gender. One has to learn and remember gender of each noun although there are rules helping people to know what gender a noun is. Again the rule is quite long. If you are interested in that, you'd better read a grammar book and write out every noun with its plural form and in a while the rule will be clear to you.