Regarding what flavour to add when...
Go to a good spice/herb store...buy small grinder or mortar and pestle...
and buy a bit of everything...you probably don't need to buy table salt, black pepper or sugar...but buy a nutmeg (be sure your box grater has a nutmeg size) , cinnamon bark, star anise, some cloves, cumin seed, fennel seed, coriander seed, mustard seed, sesame seed, cardamom, curry powder, mace, saffron threads (bloody expensive stuff), turmeric seed, celery seed, paprika, chili flakes, cayenne pepper, buy some fresh ginger, fresh sage, true greek oregano, tarragon, thyme, bay leaves, rosemary, savory, horseradish, licorice root, basil, chervil, cilantro, dill oh and get some sea salt and maybe kosher salt and some pure vanilla extract
Don't buy mixes like seasoned salt, lemon salt etc...
Buy a box of tablewater crackers and a bottle of carbonated water...
Finely grind a small amount of each spice (you will have to grate the nutmeg a bit)...The fresh herbs you can basically just crush the leaves..the roots you will want to chop off a bit then dice finely...
Now make a bit of time and smell each spice or herb...then taste it (just a tiny bit...after each herb/spice thoroughly eat a cracker and drink some carbonated water...this essentially clears the palate which actually is quite involved in the smell process...
The flavour of each herb or spice is a result of the natural oils and alkaloids inherent in the plant...the different salt flavours are the results of different mineralological contaminants...
Just for reference the flavours that are oil based will be much more cloying that is, they will tend to remain longer on the tongue so you may have to swish the soda water around a bit...they also tend to be very dominant in foods cooked with them because of this...
Each spice or herb will have a number of qualities...sweet, sour, bitter, fullness, heat (hence the very small amount...

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You will be amazed at how little you knew about which spice tasted/smelled like what...
Once you have done this try to notice in foods you eat what spices/herbs are there...
When I am cooking oddly enough I cook as much to smell as to taste...I will almost always ask myself what smell I want in the dish because the two senses are so closely related...
As to how much of each spice or herb to add...well thats tougher...You could and will of course experiment...and the experiments will go horribly wrong at times...

ask any good cook...
Look at a lot of recipes and see how much of a given spice or herb is called for...or do like I did today and ask if anyone has used the spice can recommend a base quantity then start with less than that and gradually add more until it suits you...remember you can always add more of a given flavour...it can be damn near impossible to remove too much of that flavour...
PS: a caveat dont sniff too hard...snorting up a bunch of Wasabi can be hazardous to your health...
By the way you can also begin to learn a lot about plants doing this...a lot of plant families have characteristic volatiles...

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