One problem with estimating taxes ahead of time is that they have an annoying tendency here to make changes to the current year's tax rules in November which are, of course, retroactive for the entire tax year. But you get used to that after a bit. <g>

They have a "no obligation" tax estimator form online at:
- oops, I see I have yet to post enough to be trusted to post an URL*
which is probably a whole bunch easier than trying to calculate it by hand, as there are lots of weird little factors to take into account.

Don't forget, too, that they don't withhold any income taxes from your paycheck. You'll be expected to pay off whatever you owe over a period of several months (monthly or quarterly) after your first filing in May 2009.

The good news is that there is LOTS of stuff online for estimating your taxes, filing your return (not the first time, though) and making your payments.
Cheers,
Bev

* To find the tax calculator, go to the Service-Public website (just add an .fr to Service-public), then click on Impôts, then over on the far right there is a link to the site to calculate your tax based on last year's rules.