Dont know where to start to become a marine biologistWatch
if you want to become an expert on plankton then you would go with the micro option... etc
However, that doesn't mean you shouldn't go for it. But you should make sure your degree gives you a solid plan B. I would recommend a biology degree, perhaps one with the option to do marine as an option in the final year. You might find you change your mind when you are at university and biology would be a solid choice with other transferable skills. I would usually recommend a general degree like biology over a more specialised degree like marine biology or zoology.
For degree courses, most Marine Biology courses have a relatively broad outlook and cover all these relevant areas and then give you scope to specialise more in different areas. This is usually not species-based but more along the lines of "deep sea" vs "tropical" vs "benthic" etc. But you can of course do some specific research on some particular species or what have you (which might be whales - I did see some research being done at Stanford about the role of whale excrement in the ocean nitrogen cycle, with particular reference to the Gulf of Mexico).
You don't need to do a degree in Marine Biology specifically to go into the area - a more general Biology or Ecology/Conservation/Zoology oriented course, or an Environmental/Climate Science type course would also be suitable. However these of course won't use the specific frame of reference of the marine environment, so you will need to extrapolate the results to those ecosystems and environments yourself - whereas a Marine course will focus on that from the get go. You will also get Marine Bio specific lab experience (like boat work quite often) in a marine course.
Some specific Marine Bio courses I'm aware of that seem worth looking into are Southampton, Exeter/Cornwall Campus (and trust me, coming from me this not an overrating) and Plymouth. I'm sure there are others though, and as above even courses that don't have a specific marine focus may a) have some marine biology options and b) still provide a suitable background to going on to a marine focused PhD etc.