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.
