Without seeing a live site it’s hard to comment.
Based on your comments it seems that tables are involved. If these are not being used for actual tabular data but are being used for layout then I would recommend a complete rebuild.
That’s a lot of questions, and it might be better to learn this stuff yourself from scratch cause to go into it in detail could fill a whole book. I suggest this book, it’s totally worth it: http://www.abookapart.com/products/responsive-web-design
Not saying we can’t help you here, but it’d be easier if you have more concrete questions.
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*May or may not contain any actual "CSS" or "Tricks".