A typical terran ramp defense consists of two supply depots and a barracks. The depots act as doors that you can lower to allow your units to go through, or raise to prevent the enemy from moving into your base. Players sometimes lift-off their barracks instead, but this can be riskier as landing takes a bit longer than raising a supply depot. Later on you'll need a couple missile turrets to detect cloaked units and provide additional protection against aerial threats.
Terrans have a very solid ramp defense, especially with siege tank support.