This is a good question, you might get different answers depending on who you ask. If you want to get really good at drawing you should draw anything and everything, however that doesn’t work for some.
One lady in my art class last year was very frustrated with herself as she was trying to draw animals. I asked her what she really wanted to draw and she said she didn’t know. After asking her what her hobbies were and what she did at weekends it turned out she liked boats and water. I suggested she draw boats and water and suddenly a light bulb switched on.
Always draw what inspires you. When we draw things that we like, enjoy observing and have an interest in, we feel motivated to draw and paint. The chances are that we are going to be good at drawing the things we love.
If you are not sure what to draw then I would suggest going outside and just observing, see what takes your interest. Some of us haven’t really taken the time to stop and observe anything in any detail as we are so busy rushing around. Find your interests and follow them and you will enjoy drawing even more.
There are specific things that I do suggest people draw for learning certain things, I will be writing a post about this in the near future so watch out for that one!
Also see Where do Complete Beginners Start?