Are talking at runtime? In when your are designing your report?
If it's the second one, modify your report and select the field you want to change and from the menu go to format->font and change the font.
If it's at runtime use two text boxes (one on top of the other ) one being regular font and one being bold and in the Print When put your condition as to when you want the bold one to print, and reverse the condition for the regular font one.