make UILabel's text bold

If you want to retain the system font and make it bold:

[infoLabel setFont:[UIFont boldSystemFontOfSize:16]];

Try

[infoLabel setFont:[UIFont fontWithName:@"Arial-BoldMT" size:16]];

It may also be worth checking if the font you're trying to use is available on device

Using the GUI in Xcode select the label then go to the Attributes Inspector. One of the options is Font. Click on the font icon (not the up-down arrows). In the popup that appears expand the Font ComboxBox. Under the Bold System section choose Regular.