Debug on the menu....then "Step into" and keep hitting F8 to go line by line....
the "hover the mouse over the variables" feature works then too... in the previously executed line to see what the variables are...very handy
you can drag the little yellow arrow point over lines of code if you don't want them to execute or if you want to repeat a line, etc
check out the debug menu for more options...like run to cursor..handy if your error is after a big loop or something and you don't want to step through it all