The way that you get Spotify Premium for the first few months free is to sign up. The standad premium subscription includes different things like unlimited skips, downloading music, and a Hulu subscription. You can cancel when you want.
The way that you get the premium Spotify is to login to the account page, click subscription, click get premium and enter your payment information. The cost for the upgraded plan is $9.99 per month. With this plan you won't get interrupted by ads.
The cost for Spotify Premium for the year is about $120.00.