Learn Russian: Мой брат — строитель.

In Russian, the present tense of the verb 'to be' (am, is, are) is skipped. When you link two nouns together to say what someone or something is, you replace the missing verb with a long dash. So this sentence literally translates to: My brother — builder.

Why is there no word for 'a' before строитель?

Russian does not have articles like 'a', 'an', or 'the'. You just use the noun строитель (builder) on its own. The English translation adds 'a' because it is required for correct English grammar.

Would I still use Мой if I was talking about my sister instead of my brother?

No, you would need to change it. Possessive words like 'my' must match the gender of the noun they describe. Because брат (brother) is masculine, we use the masculine word Мой. For a feminine noun like sister, you would use Моя.

The word строитель ends in a soft sign (ь). Is it masculine or feminine?

Nouns ending in a soft sign can be either masculine or feminine, which can be tricky to memorize! However, most nouns for professions that end in -тель (like учитель for teacher or строитель for builder) are masculine.