Ahh that's a good one so you don't actually believe you can do it at all.
Well do you have anything specific you are trying to manifest. Being too vague will just have you floating around doing nothing.
You need something that has measurable results.
Also don't get caught up in the term "manifesting" it's really just a buzz word people use in this community.
What it really is is taking a thought or an idea from your head and turning it into reality.
If you have a specific thing you want to manifest let me know and I'll be happy to help you
Tristan