I start my menstruation on 19 jan when am I suppose to ovulate and be sure of being fertile

Nobody can answer that... you HAVE to take and keep a basal temperature chart for several months before you can find out what day of your cycle you ovulate.

It could literally be on any day of that cycle... until you do that chart... what day is anyone's guess... some women ovulate DURING their period for example. Women as a group are very inconstant on what day of the cycle that is... they are only fairly consistent to when THEY as an individual do it month to month.

Smoothy is right!

I suggest this site. It is a huge help in tracking everything, your period, your dicharge, sex, and your temp.

On top of that, they offer a free educational 'class' (emails) that educate you on how your cycle works. Its awesome! I learned a lot from it!

Ovulation Calendar and Ovulation Chart - Fertility Charting (http://www.fertilityfriend.com)

I agree with the above. Track your cycle for a few months. There are also ovulation kits on the market - you could buy one and read the package insert. It might be good to do the tracking and when you think you know when you're ovulating, do the test to confirm your tracking results.