Magic the Gathering Noob Here

So I recently got into magic, and after some kitchen magic with some friends, I am browsing the forums and whatnot for any information about the game. After looking at all that I am starting to realize how expensive this game is. I really just want to play causally until I get into a much better financial situation (aka get out of college)
Is there anywhere I can acquire cards (like bulk commons) for very cheap or even free? I continuously find people on the forums who say they have so many bulk commons/uncommons that some people end up throwing them away or just let them collect dust. Should I ask around my local card shop or would that be looked down upon? Is there websites where people give useless cards to newer players?

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mtgcardmarket.com/
Only play Commander, pauper and draft. Everything else will eat your money and end your will to live. Buy the commander precons. They're efficient.
Buy stuff from your local shop if it's TCG player prices +- 20%. Don't buy if they're overcharging you by more than 20%. I'd recommend MTGcardmarket, above, for singles. Bulk commons and uncommons cost pennies online anyway, you can literally order hundreds for less than €10
Always be mindful of the power level of your playgroup, this is why multiplayer formats are useful. If you become too powerful and curbstomp your opponents every game in duels, they'll get frustrated and stop playing if they're fairly new. In multiplayer games this isn't as much an issue, but it still can be if you fill your commander deck with infinite combos and the like. Focus on having fun, not winning. People who focus on winning will always either spend too much on the game or ragequit because they can't afford to spend too much on the game.

Draft (cube notwithstanding) is arguably the most expensive format.

I agree, though bear in mind as a newbie it's boosting your cardpool while you do it.

for someone who doesn't have any cards this is way to build his card pool, play on a relatively even playing field with other players, and without prohibitive cost to enter, even if the cost over time could eclipse what you would have spent on a legacy deck up front

Thanks for the suggestions, I will consider these sites. Should I invest in a pre-built commander deck released by wizards to get into the commander format?

I have a doubt. Can I tap an already tapped permanent? Is it possible to tap a permanent with pic related and in my next turn, to tap it again even when is already tapped?

no, it has to be untapped to tap it

Another doubt. Sorry for bad English. If my oponent and I control pic related. Which tidal triggers it's ability first?

You have the wrong word, by the way. Instead of "Doubt" you should be using "Question"