Learn Russian: Сестра не здесь.

It ends in the letter -а. In the nominative case (the basic dictionary form you are learning now), nouns ending in -а or -я are almost always feminine. It also helps that it means "sister", which is a naturally feminine word!

Where is the word for "is" in this sentence?

Russian usually skips the verb "to be" (am, is, are) in the present tense. Instead of saying "The sister is not here", you literally just say "Sister not here" (Сестра не здесь).

The word здесь ends in a soft sign (-ь). How do I know if it is masculine or feminine?

You don't need to! Здесь means "here" and is an adverb, not a noun. Only nouns (words for people, places, or things) have a gender. You only need to look at endings to figure out the gender for nouns like сестра.

Why is не placed exactly where it is?

In Russian, the negative word не (not) always goes directly in front of the specific word it is negating. Since we want to say she is "not here", не goes immediately before здесь.