for anyone interested in playing yugioh on the computer for free...no download you can checkout http://www.duelingnetwork.com/. Afaik this game is update with the newest cards and rulings if you haven't played in a while (6 or more years) there's a couple things you should catch up on.

Synchro summons
-A synchro monster looks like this (white card omg :o)

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XYZ Summons
-An XYZ card looks like this



Last but not least
Ban list
Konami tries to keep their players happy by banning or limiting some cards in the game to stop people from making "cheap decks" so the bad news may be you wont be able to make the same deck you used 7 years ago because some of the key cards may be banned but that's not always the case. Here's a list of the banned/limited cards http://www.yugioh-card.com/en/limited/

Let me tell you how the flash game works:
DP= draw phase
SP = standby phase
M1 = main phase 1
BP = battle phase
M2 = main phase 2
EP = end phase

You click on each phase to advance to that phase. You cannot activate any cards during the DP or SP unless by card effect. When a card is activated, then it is implied that you are in your M1, BP, M2, or EP, all respectively.

To lower your life points, enter the number of life points you lost in the life points box in the middle of the screen. make sure the sign is on the negative. to increase you life points, then do the opposite with the sign (just click it) and enter the number in. everything else can be seen from highlighting your cursor over the card/deck/ etc. (borrowed this info from a friend)