How to enable Salesforce lightning

Just 3 easy steps:

1. Enable custom domain. Check out this trailhead if you don't have a custom domain.
2. Goto Setup - Lightning experience.
3. Turn On Lightning experience. Click on your name on the right hand corner and select 'Switch to Lightning experience'. Voila!
4. Optional 
1. Rollout Strategy : Create a new Profile or permission set and enable 'Lightning experience for the selected pilot users.
2. Turn on 'Account Insights' to see the intuitive component on Account detail page which provides the Sales rep with the latest News for that Account to help them impress the customers . More on it later.

If you want to read further then you can refer this trailhead.