Learn Russian: Мой парень — студент.

In Russian, the present tense of the verb 'to be' (am, is, are) is usually dropped. When a sentence links two nouns together, like 'boyfriend' and 'student', a dash is used in writing to take the place of the missing word 'is'.

How do I know that парень is masculine if it ends in a soft sign (-ь)?

Nouns ending in a soft sign (-ь) can be either masculine or feminine, so you often just have to memorize their gender. However, because парень means 'guy' or 'boy', it naturally refers to a male and is masculine. That is why it pairs with the masculine word for 'my' (мой).

Does парень always mean 'boyfriend'?

Not always! Its basic meaning is 'guy', 'lad', or 'young man'. But when you combine it with a possessive word to say 'my guy' (мой парень), it is the most common way to say 'my boyfriend' in Russian.

How would I change this sentence to say 'My girlfriend is a student'?

You would need to change all the words to their feminine forms. 'My girlfriend' is моя девушка, and a female student is студентка. The new sentence would be: Моя девушка — студентка.