Gst registration for freelancers

Greetings,

I am a freelancer and my turnover does not exceed 4 lakhs. So till now I did not have to register under current rules under Service or any other tax only had to pay income tax on my income. 

But with GST in which I heard that even if the threshold limit of Rs. 20 lakhs is not crossed, one could still be under GST and mandatory GST registration. 

Now most of my services are either within my city itself as I do website development or I build websites for outside India customers who pay me via Paypal. 

But I do have one customer who is outside my state. 

Now my query is this : 

a) If I do website development for customers within my city and my turnover is < 4lakhs do I come under GST?

b) If I do website development for customers within my city and foregin customers outside India and my turnover is < 4lakhs do I come under GST?

c) If I do website development for customers within my city, one customer outside my state and foregin customers outside India and my turnover is < 4lakhs do I come under GST?

For C I think if its interstate service its mandatory thats my reading of the law. But according to me if its a or b then I would not come under GST. So am I right? Or under any other category its mandatory to register under GST?

And what are the GST rates I need to collect? From what I read I feel if I have to charge GST then it would be at 18%. Am I right?

Thanks for your help in advance.