Starting a starter... more questions....

- is that smell "normal" ? 

- how do I know if my starter is "happy". how do I know is it's "hungry" or overfed (can it be overfed)? 

That's mostly normal, although the acetone odor develops when the starter is underfed, so feed your starter now.  Different people perceive odors differently, so you will see terms like "yeasty", "fruity", "winey", "beery", or other descriptors.  A rule of thumb about minimum quantities to feed is to discard half of the starter and feed enough to at least double the amount of starter.  One example: assuming that the starter consists of equal weights of water and flour, if 50g of starter is on hand, discard 25g and then feed with 25g each of water and flour.  You may see this expressed as a 1:1:1 feeding, denoting the relative weights of the starter:water:flour.  Some people prefer a 1:2:3 feeding regime.  The variations are almost as numerous as the bakers using them.  It is possible to "overfeed" a starter, insofar as short-tem use is concerned.  However, with enough time, a 25g starter fed with 25kg of flour will eventually use up all of the flour.

- does the frequency of feeding depends on the temperature ? (I live in Singapore which has a pretty hot climate - 25C/77F is considered a cold day, and 30C/86F is absolutly normal). Does air humidity matters ?  

Temperature and feeding intervals are very closely related.  By the way, your temperatures are right in the middle of the ranges preferred by starters.  The warmer the temperature, the faster the starter organisms consume the food and the sooner they need to be fed again.  At the temperatures in your location, a starter left at room temperature might need to be fed as often as every 6 hours.  That might stretch to 12 hours at the lower end of the range.  Feeding frequency is also affected by the starter's consistency.  A drier starter with the consistency of a stiff dough might only need to be fed half as often as a starter with a thinner, runnier consistency.  Humidity isn't such a big deal, so long as the starter is protected from drying out in low humidities and the container is kept clean to prevent mold growth in higher humidities.

- how much starter should one have ? do I keep a large bowl or just a small cup ?

That depends on how often you wish to feed your starter, how often you want to bake with it, and how much flour you wish to use.  Because I usually only get to bake on weekends, I keep about 50g of my starter as a stiff dough and store it in the refrigerator between bakes.  It can easily go two weeks in cold storage without feeding.  I then build it up to the quantity that is required for the formula (plus extra for storage) prior to making my bread.  Others are happy to keep theirs at room temperature so that it is available for baking whenever wanted, even though they have to feed it more frequently than I do mine.

- I know there's a feeding ratio of 1:10:10... but does it work for any measuring unit? one cup, one spoon ? 

- is the above ratio always suitable ? I've read that people store the started at differnt consistencies... why ?

Some of this has been covered in the previous answers.  The thing I will stress here is the importance of weighing ingredients, instead of using volume measurements.  It took me quite a while to make that conversion for my own baking but I wish I had done so much sooner.  Scaling recipes up or down in size is so much simpler using weights.  So is the degree of accuracy, in the sense that I can much more easily duplicate previous efforts.  All of the ratios that I have mentioned here are weight-based.  The 1:1:1 ratio, for instance, will typically produce a batter consistency, as for pancakes.  If I use that same ratio with volume measurements, I'll have a very liquid substance that soon separates with the flour settling to the bottom of the container.  That is because a volume of water weighs approximately twice as much as the same volume of flour.

- does it matter what flour do you feed it ?

In my experience, not really.  Mine is fed mostly white flour, with occasional feedings of rye or whole wheat, and it doesn't seem to be affected for good or for bad when the flour is changed.  Others report that it takes a few days for their starter to adjust after a change in flour.  Maybe mine is just the starter equivalent of a junk yard dog.  If a formula calls for a rye starter, I'll take a very small amount of my starter and feed it only rye flour for 2 or 3 feedings.  By that time, it is indistinguishable from a starter maintained from birth on nothing but rye flour.

- some reciepes call for 50% others for 70% .. other (most?) for 100%... I am pretty sure I dotn need to keep multiple jars... so... how do I get the variety ? 

Here's yet another reason for working with weight measures instead of volume measures.  Bakers math is predicated on weight.  Those percentages refer to the weight of the water as a percentage of the weight of the flour.  If the flour weight is 1000g, a 50% hydration would use 500g of water.  Or, to achieve a 70% hydration, one would need 700g of water.  If you know that your starter is a particular hydration (because you have been weighing what you feed it), it is fairly simple arithmetic to get from your starter hydration to the recipe's requirement by adding an appropriate weight of flour or water.  So no, you don't need to maintain multiple starters.

- when feeding the started do I really throw away a large quantaty of the starter ? (such a waste...)

If you are feeding between baking sessions, then you will need to discard some portion before each feed.  Otherwise, the starter will quickly outgrow any container you have on hand.  But, you can save the discards in another contatiner for use in pancakes, waffles, scones, or other baked goods so that you don't have to just throw away perfectly good food.  And, you can trim the amount of starter you keep on hand to 50g or less; that will also limit the amount of waste.

Have fun with your sourdough.

Paul