How to learn cell biology?

I'm in university as a Bio major and there's a lot of good vids on youtube that demonstrate certain processes... however, I passed my class by memorizing everything. Flashcards. Those became my BFF's... nerdy, but very efficient. Also, go to your public library and check a cell bio book out. They are horrible ( so much depth) but it's better than nothing.
Good Luck!!

Online there are a lot of cheap textbooks, I bought an AP Bio book online to help study and it is incredibly helpful.

What sort of processes are you looking to understand?  miRNA and siRNA are extremely advanced topics that researchers have yet to completely understand.  

If you're looking for a good high-school level textbook on biology in general (which will also cover DNA pretty indepth including your transcription, translation, methylation, post-transcriptional modifications, splicing, etc. ), I recommend the Biology textbook by Campbell (best biology textbook ever imho).  

For siRNA and miRNA specific material, I'm not too familiar with any books that deal exclusively with the topic.  You can however find lots of scientific review papers that deal with the discussion.  Google scholar is your friend here.  eg:  here's a review paper I dug up with 5 minutes of searching: Ramaswamy, et. al - "siRNA: A Guide for RNA Silencing"

Good luck!

(Here's the bit on "don't remember anything from class")

When I took AP Biology in High School, I found that the supplemental study guides helped enormously.
I personally used Baron's Ap Biology Study guide, and it worked fantastic.
In the class and for the raw information, I used Campbell 7th Edition biology. Its an expensive book.
I used a couple online sites to help me out, but
http://www.geocities.com/apbiowi/notes.html
and good old sparknotes helped me out the most.

For specific processes in transcription, I used YouTube, because I am a visual learner. Here's a playlist:
http://www.youtube.com/view_play_list?p=91C56EC3ECFFA481&search_query=TRna

(For the specifics)
You need to buy a college level book on transcription and DNA processing if you want to know about MicroRNA or siRNA. Nothing less will suffice