When looking through the store during a game;
The first item that you buy should usually be a regen item, or a gold generating item.
the second item that you buy should usually be some boots, or in certain cases like Warwick, something champion-specific like Life Steal.
After that you should build up something for your certain champion.

For instance:
If you are a AP-using champion like Veigar or Morgana, then buy a LOT of Ability Power items.
If you are a tank, then you should buy a LOT of health-bonus, health-regen, or armor items.
If you are a melee, then you should most likely buy a lot of damage and life steal, but this may change with some champions.

In general, you would need a detailed guide for each champion which will happen in time- for now, just learn things for yourself and you should do well enough, also other guides on this site can help you learn faster.