I highly recommend speed chess (5 mins), as it allows for many more repetitions. You'll learn faster what works / doesn't. Intersperse every once in a while with a slow game to rethink your fundamentals and strategy. Also, speed chess is much more forgiving. 1 mistake is not the end of the game, as time is the great equalizer. Whereas in slow chess, going down a pawn unintentionally is often a loss.
Where can I play speed chess against computer (offline/online) or people online?
There are a variety of clients that you can use to access it. I'd recommend Babaschess (also free).