Im trying to learn the chords in every Major Key and Minor Key, and i dont understand how you practice this? Is there a theory to follow which makes you think really fast like "Oh in Dm theres this and this chord" and so on..? I know theres a theory behind them all but it takes very long if you are trying to join in with someone and you have no idea what the chords are.

So should type down all the chords in every key, and then practice on every key or how should i do ?

hope i dont confuse you with my english.

the easiest way to do this is to learn how to construct triads. triads are built from a root, the third, and a fifth. To figure out what chords a scale contains, write out the notes of the scale, then build triads upon those notes using the notes of the scale. If doing strictly triads, it's easy to remember that in major keys the I(One) IV and V chords are major, the ii, iii, and vi chords are minor, and the vii chord is diminished. Once you get building triads down, start adding the 7th to each chord to expand how chords can interweave with each other. Once you have 7th chords, and other chords that contain say an 11th or a 13th, you can start looking into things like V/V's or tritone substitutions.

The basics though is to learn how chords are built in one key, know the intervals that make up a major key, and then you'll be able to use that in every major/minor key.
my english isnt great either, im better with the dutch terms... but i'll give it a shot:P

For example the C major scale....

there's 7 chords:

I C major
II D minor
III E minor
IV F major
V G major
VI A minor
VII B dimminished

That order is the same in every key, I = major, II and III are minor, IV V are major, VI = minor and VII = dimminished

To know what sharps and flats you have in every key, so you know what roots you have for the chords, you should learn the circle of fifths.

Hope that helped.. :p
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Thank you for the quick answers!
This is really good information thanks alot!