best practices when setting sizes and spacing

In a previous question, I was advised against using set pixel sizes for height, width, text, etc. I was also advised against using the "align" attribute. I'm still learning a lot about html/css, so I want to make sure I learn what is considered best practices, and not just self teach myself a bunch of error prone ways. So, I've got a few questions. 1. What is the best way to set text or other element sizes if I don't just set by pixels? 2. What is the best way to eliminate spaces between text paragraphs and buttons, etc.? Right now I've got a pretty good space going between several elements and I'm not sure how to reduce that space. 3. divs. I'm not really sure what to do with these or how to use them correctly? I currently only have one div surrounding everything for this page. I feel like that's not correct, but I don't really know what else to do with it? Hopefully you wise people can shed some light on all these questions for me!