Learn Polish: Mieszkam z moją siostrą.

In Polish, you usually don't need to say words like "I", "you", or "we" because the ending of the verb already tells us who is doing the action. The verb mieszkam ends in -m, which specifically means "I live". You would only add the word ja (I) before the verb if you wanted to emphasize it, like saying "I live with my sister, not someone else."

Why do the words for "my" and "sister" end in "-ą" instead of "-a"?

The dictionary forms of these words are moja and siostra. However, the little word z (meaning "with") forces the words that follow it to change their endings. This is a grammar rule called the Instrumental case. For feminine nouns and adjectives that normally end in -a, you change that ending to an -ą when you are doing something with them.

Do the describing words always rhyme with the noun like in "moją siostrą"?

Very often, yes! In Polish, describing words (like adjectives and possessive words such as "my") must match the gender and case of the noun they describe. Because siostrą is a feminine noun in the Instrumental case, the word for "my" has to follow suit, changing from moja to moją.

I learned that "z" can mean "from". How do I know it means "with" here?