When characters speak you need to make it clear who is speaking so readers don't become confused. In order to do this, you use speech tags. Don't overdo he/she said or it becomes repetitive and boring.

You don't want your book to read like this -

"The goats are attacking," he said.

"If they surround the building then we're all dead!" she said.

"We can't let them do that!" he said

"We need weapons," she said.

Repetitive and boring. The goats might be attacking, but the way that's written does not convey action of any sort. Also, is it a single he and she or are there more characters? If you get stuck in repetitive language like that it can make things very confusing. There are different things you can do to mix up speech tags and different ways to show who's speaking.

Let's rewrite the above scene -

"The goats are attacking," Marcus yelled as he turned away from the window.

"If they surround the building we're all dead!" Kelly's panicked words filled the small room.

"We can't let them do that," David shouted while waving his arms in the air frantically.

Kelly looked around the room. "We need weapons!" 

In the second version, you know exactly who's talking and you get an idea of some of the action taking place. It's very easy to get caught up in using the same phrasing throughout a book, try not to fall into the trap.

Another mistake new authors often make is lumping speech into one paragraph. Each time a new person speaks it should be a new paragraph. 

You never want to do this -

The goats are attacking," Marcus yelled as he turned from the window. "If they surround the building we're all dead!" Kelly shouted. "We can't let them do that," David shouted while waving his arms in the air. "We need weapons!" Kelly told them.

Speech is an important part of a book, just as descriptions are. Speech gives readers a more in-depth view of your characters and allows them to get to know the characters better. You have to allow your character to tell their story in their words.