Firstly, don't take critiques to heart, you will need a thick skin in any niche of the design industry. Had people known you were only 13, they may have been a little less harsh in their word choices.
Now, with that in mind, I don't think you are understanding what everyone is trying to say. There is nothing creative or original conceptually in your design.
It would be like me taking a picture of an athlete and slapping a Nike logo on it with the text "Just do it!".
You have just taken the creative elements of Apple's existing marketing material and rearranged it.
You can still use the fonts and Imac images but you need to come up with your own creative approach, your own creative headline, subtext etc.
You own Apple products right, so sit down with a piece of paper and just start writing down every word that comes to mind while you are looking at your products, this is called "brainstorming". From that, look for something original, an original thought, approach and try to run with it.
Also, as others have mentioned, have a strong visual and headline. Posters are more like Billboards and are intended to capture the target market within 5-10 seconds. Something needs to draw them in to read whatever copy you have within the poster.
ie. Here is a quick example I found on google. I have broken down the elements of the poster in the following way:
Concept
Approach uses humour to cleverly advertise the Apple product and its superiority to Vista by manipulating a commonly used cliche in the form of a health awareness notice/advertisement.
Headline:
"Remember an an Apple a day keeps Vista at Bay."
Subtext
"Medical fact was used in the production of this poster. Apples are a wise choice for a healthier life."
Copy
"This poster relies solely on proven scientific and medical fact to convey a public health message. Consuming vista daily, opposed to an Apple, leads to malnourishment and overall bad health."
Visual Element
Text/Typography with colour emphasis on the word "Apple".
Layout
Typography and layout or overall flow of design. The eye is drawn to the large text and reads down in a natural direction. Layout is simple, clean and clear and cleverly resembles that of a public health message.