Mentioning oyur Mac OS version helps at times like this
If you have Panther, go into System Preferences, click on International, then the Input Menu tab. Add a check box to Keyboard Viewer, and you might also want to add Character Palette and the U.S. Extended keyboard and play with them at some point. (I'm assuming you're in the US here...).
In older Mac OS, look for and start the Key Caps program, it does the same thing as the Keyboard Viewer.
The Keybord Viewer will clue you in on how to type things. If you press the option key, you'll notice that a few keys are highlighted with white squares. Those are your accents. So, to produce á, you would type option-e, then a.
There are other characters you'll see show up when you press option or shift-option.
You can lose the viewer once you get the hang of it, those work anyway.
The U.S. Extended layout I mentioned will give you more possible characters, but it only works with newer programs that can handle Unicode. Likewise, the Character Palette will show you some characters that don't necessarily work in all applications. For French, the normal US keyboard should do all you need.