Hey M.Hanako,
You're posing an interesting question, the answer to which is vast and broad.
I would also like to put forward the notion that there are in fact very few "staples" for EDH, especially at the casual level. EDH is inherently a casual format and a lot of what people like about it is that it allows them to play their pet cards, cards that they love or are nostalgic for for example, without having to worry about if they are necessarily the "correct" or most optimised choice for their deck or strategy.
Generally what you will find for EDH decks is that there are certain actions that any given deck will want to be able to perform, but the specific card choice will vary from deck to deck and player to player. For example, any given EDH deck will want to include ways to ramp, ways to draw cards, ways to win, ways to interact with opponents, but how a deck does all of these things will be hugely dependent on the colours the deck has access to, the commander the deck employs and the strategy the deck is looking to use.
For example, a spellslinger deck, a big stompy deck and an aristocrats deck will all want to draw cards, but how they will do it will be different. Spellslinger will include things like
Brainstorm, stompy will include things like
Garruk's Uprising and aristocrats will include things like
Grim Haruspex. As such, it's difficult to define a "card draw staple" and a lot easier to determine actions your deck wants to take and finding ways to do that that fall in line with your strategy.
A good place to start when building any EDH deck is to outline your ratios of cards. A solid starting point would be something like:
1 Commander
37 lands
10 ramp spells
10 draw spells/engines
10 spot removal / board wipes
5 utility cards (these could be things like graveyard recursion, graveyard hate, mana sinks etc.)
27 cards dedicated to what your deck wants to do.
These numbers will change depending on your strategy and will be tweaked with play testing, but this is a solid place to start. Also, it is often the case that one card can fulfil multiple roles, which frees up more room for other areas depending on your meta.
As robort points out. EDHRec.com is the go to resource for all things Commander. Under the 'Commanders' menu, there is an option for each colour combination. Selecting a colour or combination will take you to the home page for that colour/combo. On that page it will show you all of the possible commanders (ordered by popularity) and all of the "staples" for that colour / combo. Having a look there should give you a good idea of what cards you might want to pick up. Additionally, as robort says, there is a 'Cards' menu option. The 'Top Cards' option from this menu shows the most commonly used cards across the format, which would be another great place to look.
With the extensive history of MTG, the vast array of different play styles and archetypes, and the inherent nature of EDH being a format of playing what you like, defining a list of staples for casual players is very difficult. Have you decided on a strategy/archetype/commander/individual card you are looking to build your first deck around? If you have, this will give us a little more direction in recommending cards that would be good for that deck, but not so narrow that they wouldn't be useful in other decks too.
I hope this has helped a little. Good luck with whatever deck you decided to run with.