Finding good Starcraft 2 Terran counters is hard. In my opinion, the Terran race is by far the hardest race to counter in Starcraft 2. They have a lot of balanced units and can be pretty tough to beat
Below, you will find a Starcraft 2 Terran counters list. What I have done is list each Terran unit, and then next to it have listed what unit you should build in order to defeat the Terran unit in question.
The table below does not list what Terran units are strong against. Please see the second table for that information. The list below will tell you what to build when fighting against Terran players.
For example, in the "Marine" column, units under each race heading tell you what to build in order to defeat Marines. In the "Protoss Counter" column next to "Marine", you will find the Sentry and Colossus. Both these units do very well versus Marines.
Note: All counters are relative and fluid. For example, a single Colossus will die easily to 12 Zerglings if caught in the open field. However if your Colossi have force field support and Zealots to tank for them, then Colossi can quickly melt away Zerglings. This is just one of many examples, so take the charts with a grain of salt.
As another example, early in the game, a single Stalker will man-handle 2 Marines. Late in the game, post combat-shields and Stim-pack, Marines will dominate a Stalker. Upgrades can change list as well.
With that said, on with the Starcraft 2 Terran Counters list (
Terran Unit Weaknesses):

In this second table, you will find what each Terran unit is strong against. This is for Terran players to use. You should see what unit your enemy is building (look in the columns sorted by race) and then build whatever Terran unit is strong against that.
For example, on the "Marauder" line, it is listed as countering the Roach. If your enemy is Zerg and builds Roaches, you should build Marauders. This chart details
Terran Unit Strengths.

With these two tables detailing Starcraft 2 Terran counters, you should be in a good place both to win while playing Terran and when playing against Terran players in Starcraft 2.
If you are really struggling versus Terran players. I would recommend printing out these charts and placing them somewhere near your computer. You could then reference them whenever you get beat by a particular unit so you will know what to build for next time.
Learning all of the Starcraft 2 Terran counters can take some time, so be patient. With practice, this will all come naturally and you will know exactly what you need to build in order to win games.