str = str.slice(0,1).toUpperCase() + str.slice(1);
If you want to convert more than one letter then you might want to use the following. As examples if you wanted the first 4 characters uppercase use (0,3) and then (4) and if you wanted the first 2 uppercase you would use (0,1) and then (2)
str = str.slice(0,3).toUpperCase + str.slice(4);
Remember that an array starts at zero so always take one from the numbers you want.