if you are using a window application then you will use a label to display the text so simply set the Bold property of the label to true.
this.label1.Font = new System.Drawing.Font("Microsoft Sans Serif", 8.25F, System.Drawing.FontStyle.Bold, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
if you are using web application then simply append
tag with your string like this
string s = "<b>Monthly Modeling</b>";