I 'd like to study for a PH.D in psychology, what should I do to choose a reaearch topic from so many research topics? I only ensure that I'm interested in social cognition.
You can select the director by evaluating the rankings of publications and the impact of researchers who are dedicated to the topic of your interest and are closest to your university. To do this, you can use Google Scholar. I hope the advice will serve you, best regards.
This is a very important question. Best advice I can give you is to
1) make a brainstorm with the head of the/your research department (or other senior researchers in the field)
2) out of all the good ideas, remove any that is not very feasible. This is where you choose how depressed you will get when you have spent 75% of your PhD time to do 25% of the work. Plan smart, so you do not have to work hard. Choosing what seems an 'easy topic' does not make your PhD less valuable to you, your career or your peers. It will just make it more pleasant to do the work.
3) out of the remaining ideas, take your time alone to feel what actually speaks to you. What captivates your attention? You have to want to spend a LOT of time with this subject. Its kind of like a relationship. If you are feeling a bad itch or having second-thoughts on the first couple of dates, its most likely not the one for you! But if you feel like cuddling up with this topic in a remote book-laden cabin in the woods in front of the fireplace for weeks: Its probably the one for you! :D
Alex makes a great point, you can't go wrong with his recommendations. Here is my recommendation:
Start out with a question. What is something you want answered? From your question, start exploring what exist to answer that question. From there you'll have a better picture of what research has been done and more importantly what is missing from the research. From there start brain-storming where do you want to go with the study: Contribute to existing research or fill in a gap on the research.
You can make it about something you know a little bit about or a lot about, but either way by the time you're done, you will have spent an exhausting amount of time and energy on it.