Answer:
If you already know the present indicative tense (that's the simple present tense that students are taught first when learning the language), then all you need to do is use the third-person singular form of the verb as the affirmative informal command.
Step-by-step explanation:
present tense examples ....
-ar -er -ir
1st person singular: canto como escribo
2nd person singular: cantas comes escribes
3rd person singular: canta come escribe
Your friend says to you: Canta más fuerte.
Your mother says to you: Come el brócoli.
Your teacher says to you: Escríbeme cinco oraciones.
Of course, there are plenty of exceptions (irregular forms) -- all of which must be memorized. :(
Here are just three common examples:
poner .... pongo , pones , pone → pon
decir ... digo , dices , dice → di
hacer ... hago , haces , hace → haz