The knowledge and use of spices determines the skill of a cook. Artistic usage of the simplest spices is vastly preferable to the misuse of the most expensive and rare.
Spices are an investment. Whatever the size of this investment, it would be silly to waste it. Buy spices in the amounts that you will use them in. It doesn't save money to purchase in bulk if those spices go weak on the shelf. Try to keep them in airtight containers, an in the dark as much as possible.Here is a list of the spices that I use, and the quantity that I purchase. It is here so that you can have a list of many spices, and an example of how to understand your own usage and use it in purchasing quantity. My most common quantity is about a shot-glass of any spice.
I do recommend purchasing spices in whole form and grinding yourself if possible. This will give you the best quality.
If you are just starting out and need to build your spice rack slowly, begin with a basic set. Build your rack by deciding what kinds of cooking you will be doing most, and buy the appropriate spices. Here are some suggestions:
Basic Needs: garlic, salt, pepper, cinnamon, vanilla
Baking Set: allspice, cloves, nutmeg, cinnamon, vanilla
Expansion set: chives, dill, parsley, rosemary, mint, ginger, tarragon, sage
Premium Herb Set: thyme, bay, marjoram, savory, chervil
Italian Set: basil, oregano, garlic, onion flakes, bell pepper flakes
Mexican Set: chili Powder, cilantro, cumin, garlic, crushed red pepper, oregano, paprika, onion flakes, bell pepper flakes
Middle-Eastern: allspice, cumin, turmeric, cloves, mustard seed, coriander, cardamom, cinnamon, nutmeg, mace, crushed red pepper, fenugreek
Seeds: celery, caraway, fennel, poppy, dill, mustard, sesame, anise