Unregistered under gst

If you are unregistered you can nither collect nor pay any tax. If you are not registered you are not required to file any GST Returns.

it's mean turnover below 20 lakh is tax free

It means you are tax free till you are unregistered​.
As soon you register yourself you will have to pay GST and file returns regardless of turnover limit

and what percent of turnover should I pay under composition scheme under gst Act

Rates in composition scheme are:-
1) 5% for outdoor catering and restaurants.
2) 2% for manufacturers
3) 1% for others

Composition scheme is not available to service providers with the exception of outdoor catering and restaurant.
In composition scheme you can neither take credits on purchases nor charge(collect) GST in bill that means you have to pay GST from your own funds.

What is the nature of business of the assessee?

CA ALTAUMASH ZAFAR JI is correct.

if I purchase a taxable goods from unregistered dealer.

should good receiver pay tax under reverse charge or good outwards? is there any option to utilize the input credit? like tnvat purchase u/c 12.