Before I pick up my brush or palette knife to start a new painting, I have to figure out what I'm going to paint first. I've painted from: still lifes, a single photo, several photos combined together and have painted outdoors on site (plein air). They all have benefits and drawbacks.
These are fun to set up next to your easel with some good light. I have clamp on lights that I can aim to adjust the lighting, plus a south facing window which gives natural light. My still lifes often have flowers which will blossom and wither in a few days so there isn't a lot of time to start and complete a painting. Beware that with tulips, they will move around even in a few hours!
This is the easiest method in that: the light won't change, the flowers won't fade, and your angle to the subject won't change as you move around. You can also draw lines over your photo to place crop lines or to move something around.
This one is harder, so I don't recommend this to beginners. Once you are comfortable establishing consistent lighting to your paintings, you can get away with this approach. You can do a sketch first, and is highly recommended. You can use the sketch to figure out your composition and how to combine the different pieces you want to incorporate. I however, often just jump in and start placing things. This sometimes gets me into trouble but I really like the adventure of keeping painting an unfolding adventure rather than a thoroughly preplanned trip.
There are many things to keep in mind with plein air. I did a bunch of reading about it before I did it and there is a lot of great advice out there. However, after several plein air sessions, I discovered something that I hadn't seen mentioned. When painting out of doors, there is normally much more light, than indoors. So my paintings looked great outside, but inside they were faded looking and not nearly as vibrant as the ones I painted in my studio. Thus I learned that when painting outside, I had to exaggerate the colors I saw and paint both brighter and lighter.