Some studies have shown that we can develop insomnia by doing work on our beds. The recommendation, then, is to utilize your bed only for two things: sex and sleeping. (That is current medical advice.) I take that to mean that our mind subconsciously comes to associate a location with an action. If you're having trouble focusing, establish a "study spot." Maybe it's a table in your apartment, or maybe it's a library on campus. Go there, and - at least in the beginning - put most of your focus on making sure that you just do work and don't mess around. Eventually you may find that simply being in that place increases your focus and puts you in an industrious mood.
As food for thought, Pascal once said (roughly) that habit becomes automation, and automation leads the mind. The hardest part is establishing a good habit while avoiding bad habits. Once you have the habit (routine) down, everything else may come on its own with very little effort on your part.
The key is to be disciplined, particularly in the beginning. It's like strength training - in the beginning it's very difficult, painful, and disappointing, because you invariably will not be able to meet your expectations the first couple of times. Keep at it, and don't beat yourself up if you fall off of your routine here and there. What you are doing is building your discipline and your ability to focus. The material you learn in class may feel trivial, but those two skills are things that you'll need to get into and through medical school. So if for nothing else, keep that in mind and don't feel that you're wasting your time.
As for how to study, I'm afraid that varies class by class and from person to person. Some people do well with study groups; others hate them. To aid in memorization some people do well by drawing things out in a concept map; some create mnemonics; some create stories; others just read it over and over until it's ingrained in their mind. If a test bank is available to you then studying off of old exams can help, particularly if a professor tends to keep his test formats the same.
You recognize your shortcomings and want to change. That's good and admirable. No one bit of advice in this thread will be the answer, but consider the ideas, try certain things, make them your own, and keep at it. Best of luck!