you can check below plunker for example , its simple directive with shared scope , for reference you can check this link https://docs.angularjs.org/guide/directive and my template is as below
<input type="radio" name="myradio" ng-model="myradio" value="Yes">Yes</input>
<input type="radio" name="myradio" ng-model="myradio" value="No">No</input>
<input type="text" ng-model="mytext" ng-show="myradio==='Yes'"/>
In example , you can access value of enter text from variable "mytext" , you can set default value of radio button by setting value of variable "myradio" to Yes or No , I had set this value to No in controller.
This directive can be improved by making this isolated scope and you can use it multiple times , you can try that on your own.
Please find plunker link below
https://plnkr.co/edit/0ZsNelh2DWUwMSRmvgfa?p=preview