Where to start?!

Its a good idea to sketch out the design on paper first, so you know what you're making first. When you start on a map, don't put in all of the props and aesthetic details. Make the whole "skeleton" of the map (just the brushes and objects that are essential to the map layout) using the dev textures (enter dev into the texture filter and use the plain grid textures). This lets you finish the whole map very quickly and get to testing the layout. Once you have some testing, you can make tweaks easily since there aren't hundreds of aesthetic objects cluttering your map. After you've done extensive testing and are happy with the layout, you can go through the whole map and decorate it to your hearts content.

It's also worth considering what gametype you want to make. CTF often has much smaller maps, KOTH and Arena revolve around a single point, 5CP can be linear or displaced... etc. It helps you to define the general size and shape of your map while sketching it out, as well as where you want the players to go.

Usually spawns, forward or no, can be set aside until the main combat areas have been mostly worked out. It's fast and easy to construct a simple room and think about its exit placements, which have more effect on gameplay than the interior.