"I have a sister."
Translation:Yo tengo una hermana.
As a rule of thumb, if it ends in an '-o', it's usually I (verb). Tienes ends with an '-s' which usuallt means that it is the (informal)you version. Tiene is the version of the verb used for HE/SHE/(formal)YOU has. And, as Alex486337 says, tenemos ends with '-os', meaning that it is we (verb) and tienen ends with '-n' meaning that it is they (verb).
In that case, is it possible to shave off "Yo" and have it be "Tengo una hermana"?
For verb conjugations, I'd highly recommend looking up "Que hora es?" A series on YouTube, also called "Spanish for beginners" by a channel called AIB. It's just one guy in a classroom but he really cleared up the conjugations that I just couldn't understand. He uses a pretty common chart but the way he explains it is fantástico.
Sorry that i cannot provide a link, I don't have accessto YT to get it at the moment, but if you're having trouble with how to end verbs properly, look it up!
"Un" is used for males and nouns that are considered "masculine" (such as "hermano"), while "una" is used for females and nouns that are considered "feminine" (such as "hermana"). Most words ending with the letter "a" are feminine, while the majority of words ending with "o" are masculine.