"I have a brother."
Translation:Yo tengo un hermano.
This is just one of those irregular verbs that behaves a little differently in the first person. Every language has words that live outside the rules a little, this is one of them. If you look up "Spanish irregular verbs" you'll see a list of other ones that behave similarly.
English has a lot of irregular verbs too, but the irregularity is usually crossing from present tense (happening now) to past tense (already happened). Usually you indicate something is in the past tense in English by adding -d or -ed, so I cook becomes I cooked, he walks becomes he walked, she talks becomes she talked. But we havetons of irregular verbs that break the pattern: speak> spoke, run > ran, drive > drove, think > thought, catch > caught.
You learning tengo = I have is similar to a little kid learning it's "I ran" and not "I runned"