Now is not the time to concern yourself with length. (tee-hee, Mae West)
Write this book. Then write another. Go all out and write a third. Write well, and the book can be as long as you want, or as short. Just put the whole story in there, and leave out the boring parts.
You may end up with a novella. You may end up with a trilogy. But the very first thing you should focus on is the STORY. If you're just starting your first novel, word count requirements shouldn't even come into play. Save that for the headache of querying agents and submitting to publishers. There's a lot of work to do between now and then.
Here's a nifty trick I leanred...on Amazon, you can find novels in the same genre you're writing in. Click on the ones that have "Search Inside!" tags, and click on 'text stats'. It will show you the word count of the novel. Another place to look for word counts are on the publisher's websites, if they have submission guidelines listed, they'll put word count requirements on there. Like Forbidden Snowflake said, 80-100k is a good goal, but don't use that to inflate a story, and don't stop too soon because you've hit 100k.