I need help on whats best to say for interview questions?

hi everyone. ohk so heres my situation. i have been looking for a full-time job and ive had one interview so far and have another tomorrow and the last interview i know i did horrible because it didnt last for more than 5 minutes while others where in there for at least 15 to 20 minutes. i have a tendency of looking away and saying idk ummmm..because theyll ask me why do you think your best for this job and ill literally say idk blahblahblah. or why do you want to work here ill do the same. i just get so nervous and never know what to say...could you guys give me a thumbs up and tell me whats best to say?