The first one is correct. Their usage( present perfect/past tense) depends on the time you are talking For example if it it still morning , when you are saying it, then, the present perfect tense is correct. to miss my favourite tv show.I have woken up very early today because I don't want to miss my favourite tv show.**. (The use of the present perfect tense here, tells us it's still morning,a , ), and it implies that the TV show will be shown at some time in the morning. (Why else, would you wake up early?). or I woke up very early today because I don't want to miss my favourite tv show. ( The past tense here shows us that it isn't morning any longer; it might be noon, afternoon,etc). Although the second sentence seems correct, waking up very early to watch a TV show at noon, in the afternoon ,etc, doesn't make much sense, does it?