The easiest way to take down Terminators is to hit them with the biggest, meanest guns that your army has to offer. Lascannons, starcannons, Shokk Attack Guns, dark lances, railguns, exorcist missiles and plenty more besides have an AP of 2 or better and the strength to easily cause wounds. A similar philosophy should be adopted for close combat. Power weapons, power fists, Rending and the like will also greatly aid you in cracking open that thick armor.
If you ever find yourself in a spot where you don't have the ideal weapons for the job, there's always the old fashioned way - fire saturation. Light them up with so many guns that they're forced to take tons of saves. Even the luckiest players have to roll some 1's eventually. When taking this approach, adopt and all or nothing way of thinking. It won't help much if you devote a single unit of Guardsmen with lasguns to taking out the Terminators. If you plan on overwhelming them with sheer number of shots, go big or go home. Two, three or more units getting in close with rapid fire weapons should leave an impressive dent in all but the toughest units.
Terminators tend to arrive on the battlefield in a few ways. They'll either walk like everyone else, grab a ride in a Land Raider or most commonly Deep Strike onto the battlefield. If the Termies decide to walk, treat them like you would an AV14 battle tank. If they're riding in a Land Raider make it your top priority and destroy it ASAP, entangling the Terminators and taking them out of the game for a while. Deep Strikers are a bit harder to deal with since they're far harder to avoid but as long as their initial salvo doesn't deal your army crippling damage, you can just throw a large, expendable unit at them to hold them up in close combat for a few turns while you bring in some assault specialists to finish the job.