How you plan a map is really up to personal preference. Some people like to draw, others make rough block-outs in Hammer, and some people just make it up as they go. Find out what works best for you and go with it.
When you get to the actual map creation part, there are some unspoken guidelines. Most people will recommend starting out with dev textures and working on a larger grid size - this helps keep your layout simple and easy to adjust, and saves you the trouble of possibly deleting or moving detail work you've already done. Once you've refined the layout to your satisfaction, then you can focus on making it look good.
"When to optimize" sort of goes hand-in-hand with the previous point. Try to design your layout from the ground up so that optimization will be easier down the road. For an alpha, the easiest things to keep in mind are 1) minimizing long lines of sight across the map, 2) dividing it up into smaller sections, and 3) sticking to a 64u grid for the basic shapes. Starting with optimization in mind will save you time and headaches when you get to more advanced techniques like hinting and areaportals.