Playing as Protoss against Terran can be very challenging
, but you already know that. Terran are very unpredictable and tough to scout. You have to know what the Terran player is up to so you can counter his units. However, this task can seem nearly impossible against a Terran player. In this Starcraft 2 strategy guide I'll teach you how to beat a Terran player in a 1v1 ladder game.
To begin with, the key to winning almost any game is proper scouting of your opponent. This concept will come up time and time again in almost any Starcraft 2 strategy guide. This is why I recommend you get a Robotics Facility and an Observer as quickly as possible so you can scout your opponents base, take careful note of the units he is producing.
If you notice the Terran player producing Mauraders and Marines, build some Colossus because they are very effective against tightly grouped units. Remember you will need to build some Zealots to 'tank' for your Colossus. If you are up against a Mech Tech of Siege Tanks and Thors try and get a bunch of Immortals because they will destroy these heavily armored units. Keep an eye out for Banshees or Vikings, if you don't have enough anti-air these will decimate you. If you notice them, try and get some Stalkers to attack them. You shouldn't build a lot of air units against a Terran opponent because his Marines will be able to destroy them easily.
One more thing to keep an eye for is the early Reaper rush. To stop this strategy, build a Stalker as soon as possible. Once you've mastered the basics here you will have no problem defeating good Terran players.
You can find many more Starcraft 2 strategies and reviews of some of the most popular Starcraft 2 guides at my website.
Best of luck to you. I look forward to seeing you on BattleNet!