How do I know when to start decreasing for a mitten for myself. Where in the mitten do I start the decrease? When the mitten is at the tips of my fingers or sooner?
The general rule (it would be confusing to say "rule of thumb") is to start decreasing when the body of the mitten reaches the tip of your little finger. This site may help.
There is a serie of videos which can teach you step by step how to knit mittens. Here is the link for your specific need: http://www.knitpicks.com/tutorials/Kelleys_Mitten_Class-Hand_Decrease__D204.html . There, you can also select any other video which might be useful for you.
Enjoy your knitting!
Patience, persistence and love are the main ingredients for wonderful knittings.