You can prepare for dinner by doing whatever it is that is necessary for you to be ready to have dinner. Maybe you wash your hands. Maybe you put on fancy clothes. Maybe you set the table. It has to do with getting ready.
You prepare dinner by making it. "I prepared dinner" is about the same as "I made dinner."
Can you draw your own conclusions about difference in your other two sentences?
Alright then 'prepare for lecture' means to do everything for the lecture, such as studying, making handouts, shower, dress-up, packing and be in time.
'Prepare lecture' simple means to get ready to lecture to students by studying and researching about the lesson beforehand.
Am I right?
PS: it there are any grammatical mistakes in my composition please correct them. THX :-D