You have to take GST registration if your turnover cross 20 lakhs in year. In your case since you estimate that your turnover will be more than 20 lakhs, you will have to take registration.
Rate of tax on online advertisements is 18%. Though Adsense income is zero rated supply and you will have to follow export of services rules.
Regarding direct advertisements, you will have to charge tax at 18%.
You will be able to claim input credit of tax. Input tax credit is the tax you paid on expenses incurred to run your business.