Edu_agca - it might help if you check out Kirk's introductory lessons here:
http://www.guitarforbeginners.com/chords.html
It explains it pretty well and will get you started.
There are a TON of songs that use a simple I-IV-V interval (the I, IV and IV chords of any given key). Once you read the lesson above, you'll understand better. It's not really as confusing as it seems!