Alright, You'll first need to know how Hammer works. I'd suggest a few youtube video's for that PH4T3 has some decent start ones, but when he mentions Hollowing out and Carving, ignore it. Also,
The Valve Development communty is a good place for tutorials, they have a "My First map" guide, which will help you get through everything you need to know. For other maybe more specific things, our
tutorial section is a good place to go. Have a LOT of guides on everything from getting the best screenshots of your map, to... basically a lot of different things.
Once you are familiar with hammer (and I REALLY suggest you do because you start modding Degroot),
you'll need the Degrootkeep .vmf (which is the editable map file). Which can be found
here. MAKE SURE WHEN YOU SAVE THE MAP YOU SAVE IT AS A DIFFERENT NAME. To change the time of day, you have to change the env_sun enitity and a few other ones (fog_controller, Shadow_controller). its best too look up what these are, and how to add lighting to maps.
Hopefully this will get you started... and Welcome