Read the instructions, it' all in there. If your loader didn't come with instructions, go to the Lee website and look at one of their instructional videos. The dipper supplied with your kit is for a specific load of a specific powder. Don't try to "make do" with your dipper using other powders and guessing at how much goes in the case...
For .38 Specials with 158 gr bullets I've used Bullseye, W231, and Unique with standard small pistol primers. Unique is the most versital, and you'll be able to use it when you start reloading other calibers...