Download and unzip the Roboto font zip file
Create a assets folder in your project if you don't have one already. Am assuming you are using Android Studio, this is how to do it.
Create a new directory in the assets folder, name it font.
Open your unzipped file and copy the font style of your choice. Your font folder shouls look like this:
You can now use this font anywhere in your app like this:
Typeface roboto = Typeface.createFromAsset(context.getAssets(),
"font/Roboto-Bold.ttf"); //use this.getAssets if you are calling from an Activity
txtView.setTypeface(roboto);