Change the JTextField font and text with radio buttons
Java naming convention

You should read the Java naming convention. Class require PascaleCase, so
public class gui should be public class Gui.

Indentation and style

You should use an IDE or a tools to automatically correct any styling or indentation error. The spacing of your variables declarations are not correct and your indentation of your if-else are not correct either. 

There are plenty of good free to use IDE and tools. This goes in pair with the Java naming convention/ Java Style recommendation.

Use better names

You're not limited in size for your variable names. Go for clarity over brievty! 
This will go a long way to help you, at first glance, know what the variable is about. For the next person that read your code it will be easier too. 


private JTextField tf = new JTextField( 25);
private JRadioButton regularRadio = new JRadioButton("Regular",true);
private JRadioButton boldRadio = new JRadioButton("Bold",false);
private JRadioButton italicRadio = new JRadioButton("Italic",false);
private JRadioButton boldNitalicRadio = new JRadioButton("Bold And Italic",false);;
private ButtonGroup group = new ButtonGroup();



Should probably be : 

private JTextField textField = new JTextField( 25);
private JRadioButton regularRadio = new JRadioButton("Regular",true);
private JRadioButton boldRadio = new JRadioButton("Bold",false);
private JRadioButton italicRadio = new JRadioButton("Italic",false);
private JRadioButton boldAndItalicRadio = new JRadioButton("Bold And Italic",false);;
private ButtonGroup group = new ButtonGroup();


init

In most GUI code I've seen, you can create an init method and put all the initialization code in there. It helps to keep the constructor clean, but it's no big deals. It's what I'm mostly used too, but it's been a long time I've done any Swing related code.

Constant

If you see a value used over and over, ask yourself : Does this value represent the same thing?

Take "David" for example, it does represent the same named font in all your case. You could then have a private static String DAVID = "David" to represent the font. 

Action command

This is a special case from your previous point. Since you hard-coded the action command names, if you change it at one place you have to remember to change it in other places. 

This is a perfect example of the Java Enum class. Take a look at https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html, it's a good tutorial on how to use it. 

Using an enum you make it clearer and should be easier to use in the long run.

Other point

I don't think you need to create the Font object every time, you may use an const Font object to reduce object creation. It's a small thing that not improve really but it could help in other cases.