There are lots of Terran players out there probably because that is the race you play as in story mode.
But lots of players still do not understand how they should open most games they play against other good players. Understand that you will need to be quick when playing. I mean get this down to the last second.
Obviously you should start out by sending your SCVs to harvest and then immediately start training one after another. Always remember to set the rally point on one of the mineral fields.
Then once you have about 30 minerals saved up at 9 supply send a worker to the top of your ramp. Once you hit 100 minerals drop a supply depot to block the choke. I usually wall off my base with two supply depots and a barracks.
This should how you should start especially against zerg players who like to rush with zerglings. But I would not do it facing terran players that much because if they tech to siege tanks before you then that puts you at a disadvantage since you will be out of range.
Anyway after the supply depot is done send him out to scout your opponent. Take another SCV off your mineral line while you keep training them at your command center. With that SCV build your barracks at 12 supply.
Put down a refinery as the barracks is building but only have one SCV mining gas for now. Build a second supply depot to seal the ramp keeping out units.
Then once your barracks finishes start training marines. At 16 supply upgrade to an orbital command. Build a second barracks while trainign marines at your first to guard the wall against any units.
Then once the orbital is done saturate the gas with two more workers, keep training SCVs and use the energy on mules as part of a strong economy. Once the second barracks is done attach a tech lab.
From here on out you can transition into anything. Heck if you want to build a factory instead of the second barracks for early tanks you can build a second gas and saturate both. This is the standard terran opener.