DK's got excellent defense and power, but largely at the expense of speed. To understand how he fights, just look at his unbalanced proportions. His attacks all have completely different ranges and very specific points of contact. Those involving his feet get very little distance while his arms give him the reach of a sword-fighting character. Unfortunately, while the sword characters tend to have good sweep to their slashing attacks, DK has no real sweep to any of his A attacks, save his up-A and down-A attacks. He does major damage with major kick everywhere his fists land, but generally speaking, if the fists miss, the move does no damage, even if the rest of his arms make contact. This makes his slow, sweep-less smash attacks very difficult to execute. He's deadly at arms' length, but once his personal bubble is breached, he's at a huge disadvantage and must rely on his faster physical attacks or his B moves to free himself. His up-B and down-B moves are his only good area damage techniques, but his down-B move is pretty slow. The moment your opponent gets closer than arms' length, spin away using DK's up-B technique, preferably striking your foe as you flee.
If you're an aerial fighter, don't use DK. His aerial A moves are almost impossible to execute, largely on account of their slowness. His most useful move is his chargeable B move because it's faster, more powerful, and above all, easier to aim than his smash attacks. Consequently, I advise you to play defense so that you can charge this move as often as possible, but of course, don't let people exploit DK's total lack of projectiles by sniping you.