"I am prepared for something" means that you are prepared to face its challenges and handle the situation well. For example: "I am prepared for the worst, but hope for the best" (I believe I can cope with the worst situation, should it happen).
"I prepared for something" means that you made all the necessary arrangements to be able to cope with something, but now you may or may not be prepared to face it. For example: "I prepared for the the worst, but it never happened" or "I prepared for the worst, but it wasn't enough".
Regarding your specific question:
I: Hello. I prepared my presentation for introducing my company.
I am prepared for my presentation for introducing my company.
In the first case, you're saying that you made your presentation ready beforehand for the purpose of introducing your company. So, if you want to present it now, you can, because it's ready.
In the second case, you're saying that you are prepared to make a presentation should you need to, but it does not necessarily mean that your presentation is ready already.
Do notice, though, that in the first sentence you did not use "prepared for" (as in "I prepared for my presentation"), you used only "prepared".