Hi Suraj,
Open the form in design mode.
Add a conditional formatting for the field (let say filed1) which you want to make read only by checking a check box. To do that follow the below steps.
1. Go to the field1 properties and Go to Display tab.
2. Click on conditional formatting and the following expression.
a. my:field2 = string(true())
here field2 -> check box
3. Check the read only check box and then click ok to close the windows.