When to begin using starter?

does it have a sour smell if so you can start using it if it's been 12 days

My guideline is that a starter should be at least a week old and should be reliably able to at least double itself in size between feedings.  This does not apply to a thinner starter, but to any at 100% or lower hydration.

For reference, a cup of water weighs between 100 and 200 grams, usually between 120 and 170 grams; a cup of water 240 grams.  So, one cup of water and one cup of flour will yield a starter between 120 and 240% hydration, or more commonly between 140 and 200%.  Thinner starters like these usually don't have the physical strength to double, and tend to be underfed so they throw off hooch.  Hooch is a reliable indication that your starter is underfed.

Professor Calvel recommended a starter around 67% hydration, which is about 100 grams of flour and 67 grams of water.  If you sift your flour once, spoon it into a cup, and scrape off the excess with a flat blade, you'll have a fairly consistent 120 grams per cup.  So, 1 cup of that flour and 1/3 cup of water would do the trick.

That's a bit thick for beginners to handle, so I usually suggest 100% hydration which is equal parts of flour and water by weight.  Or 1 cup of flour (sifted, etc. as above) and 1/2 cup of water.

As to sour, there are many. many variables that impact sour.  The strain of critters you caught.  The temperature at which you are handling your starter and rise your bread (incidentlally, a refrigerator really is too cold to get much of a rise - most commercial retarders are in the 45 to 55F range), how long you let your bread rise, and the ash (mineral) content of your flour.  There is also a lot of inconsistency in what people think of as sour.  Hand two people the same piece of bread.  One wiill say it's mild, the other will say it's too sour.

A last comment - I find that I get the best results through long rises.  I don't get a sourdough taste because I added sourdough starter to the dough, the way I get chocolate flavor when I add cocoa - instantly.  I get sourdough flavors when I let the dough rise slowly.  I talk about this at more length than I can go into here at http://www.sourdoughhome.com/sour.html

-Mike