Have you ever turned to a friend to say, "let us go to the mall this afternoon" rather than let's? Of course you haven't. Contractions are a natural and normal part of dialogue, and if you're a writer you really need them in your book.
Contractions are confusing. Apostrophes always complicate matters, because they appear all over the place. An apostrophe might be near the beginning, near the end, at the very end...sure, it's easy to get all mixed up. Start throwing contractions around, and matters only get more complicated. In contractions, apostrophes have a single function: they represent the missing letters. When cannot becomes can't, the apostrophe symbolizes the n and the o.
And once you know that, you should be able to write contractions perfectly every time. You'll need to, because you need contractions. Without them, your book will sound stilted and strange. It won't flow well, and I can prove it.
"You are so silly," said Mary. "We are already there. It is just around the corner."
"We have been driving for hours," Becky answered. "It is about time."
Doesn't flow well, does it? It reads much easier, and much more naturally, like this:
"You're so silly," said Mary. "We're already there. It's just around the corner."
"We've been driving for hours," Becky answered. "It's about time."
Even outside of dialogue, you need contractions to make your words flow better. Don't forget to look for opportunities to use them, and if you leave them out try to catch them in editing. See which words can be smooshed together to create contractions, and smoosh them. They're common, they're used every day, and they're much more approachable than the alternative.
There's really only one reason why you need contractions if you're a writer: everyone's using them. Always remember to write the way people talk, and you'll be a better writer.