With regular buttons, the easiest way is to add an ampersand character to the Text property, immediately before the character you want to use:
myButton.Text = "Hel&p";
When the user presses ALT+P, the button will be activated.
This also works with labels: they will activate the next control in the Tab Order , so a label with the text
myLabel.Text = "&User Name:";
will activate the textbox after it when ALT+U is pressed.