Yes. The pop-up flash on the T3i can be used as a remote flash master. However, the system is light-based, like a tv remote, so the 430EXII's sensor panel has to be able to 'see' the pop-up flash's signaling. Indoors, with lots of bounce surfaces, this can work great. But for outdoor portraits, without bounce surfaces nearby, and in brighter ambient light, which can make the signalling harder to "see", the range, reliability, and line-of-sight requirements get stricter, which is why radio triggers are so popular.
.... or do i need to use something like a pocket wizard? cause those things are really expensive and i dont want to start using transmitters and so.
You don't have to use radio triggers, but for outdoor work, you're liable to want them a lot more. Not all radio triggers are as expensive as PocketWizards, however. If you require professional-level reliability, PWs are the gold standard, but if you're just a hobbyist shooter that can afford to miss a shot now and then, you could go with much cheaper alternatives like triggers from Cactus, Yongnuo, Pixel, or Phottix. Some of them even communicate eTTL.
ive heard that my t3i can control off-camera flash but shoot the built in one before the image is taken - is that right?
Yes. The pop-up flash has to blink out the settings for the remote flash, as well as to communicate the "fire" sync signal.
for indoors and outdoors for a begginer in flash photography like me, will the ETTL mode be enough?
Indoors yes. Outdoors, possibly not.
i was also considering a 135 f2 aswell. ...
Great lens. Love it. One of my most used. But a flash will probably get you more bang for the buck, especially if you go off-camera with it.