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Depends on what you mean by hard.
When I was learning it, it seemed bewildering until I got enough background in the subject for it to start turning into a world view. Now it rocks.
There are two general textbooks, one by Freeman and the other by Campbell, which are written at an advanced high school level. If you're going into AP biology either one would be good to read.
If you're going into college, I recommend "Molecular Cell Biology" by Lodish and friends. "Biochemistry" by Garrett and Grisham covers (not all) of the same material but takes a more thorough, rigorous approach to the mechanic aspects of basic cellular processes. You can actually download a .pdf version of Garrett and Grisham for free, so if you're on a tight budget it would be an especially good idea.
Where can I download Biochemistry? I'd like to try that.
Kainui, I can't find the download link. Perhaps the anti piracy brigade took it down. I do have the copy I downloaded here on my computer. If you tell me your email I'll send it on to you.
The pdf is all of 42.3 megabytes. I'm working on splitting it up into different docs (which you can paste back together in Adobe) but it might take me a little while. I'll definitely get back to you with it.