If you are asking this question for a pixel perfect mockup, you can skip the following paragraph. Otherwise, you MUST read it:
Except for pixel-perfect mockups, you should not create images for your textual buttons to use in production, especially given the power of CSS for web or styling for desktop/mobile/any other target platform. Because it would make your application non accessible to screen readers. Because you would need to create as many buttons as languages supported by your application. Because you would need to design every buttons and other graphical compenent in the exact same way to achieve a uniform website/application design.
Now the answer to your question:
If you use The Gimp, you just have to check the option Antialiasing in the Text Tool panel. And never resize the button after creating it. Instead re-create another one for your various sizes.