In Japanese, there are two kinds of verbs: る and う. Well, there are actually three, with the third kind being "irregular verbs." But since there are only two irregular verbs, we'll cover those really quick!
So why are there three different kinds of verbs? It's mainly because they are conjugated differently. Let's take a look at the two irregular verbs:
する (suru) = to do
くる (kuru) = to come
The regular, unchanged form of these verbs is called the "Dictionary" or "Short" form. This form is also used in casual speech, so it may also be viewed as a casual form. The importance of this is that all verbs are conjugated FROM their dictionary forms. します (shimasu) is the -masu form conjugation of する. Although we'll cover -masu forms later, I'll use a little bit here to explain the two irregular verbs.
る and う verbs have their own rules of conjugation, but since する and くる irregular, you'll have to learn these two specifically. To change する and くる to their -masu forms:
する becomes します.
くる becomes きます.
These two verbs will have their own specific forms when it comes to the many Japanese conjugations. For now, the important thing to remember is that する and くる are both irregular verbs! Whenever you learn a new conjugation or verb form, you'll have to find out what する and くる is specifically, since they don't have their own set rules. This may sound confusing, but when you learn about the other verbs and forms of conjugation, I promise it'll be easier to understand.