Many of these things will depend on the specific university you do your graduate work at. Picking that university will depend on the kind of work that the profs do, and whether you are interested in that specific kind of thing. If you have not already picked that school, do this. Get yourself to the library and get some magazines like Astronomy Today and similar. Find the issue that shows recent grads and where they went after graduation. Find the schools that send their grads to places you would like to work.
Also, google up the web page of schools with programs you find interesting. Find out what research the profs there do and see what looks like you might find that interesting. Possibly
http://arxiv.org/ will be helpful there. Once you have some candidate grad schools, find out what requirements they have, and what they look at for admission. Sometimes their course calendars are on line so you can see admissions and requirements for various programs.
Then you can start to plan what makes sense for you to study in undergrad. For example, maybe the grad school has a language requirement, and maybe you can polish that off in undergrad. Maybe a lot of computer classes will help or maybe not. Probably a lot of calculus will be good. Maybe some undergrad astronomy classes are in order. And so on. Get the course calendar for your school and pick the classes you will take.
Once you know the classes you want, then you pick the degree. Maybe you can't get the astronomy classes in the btech? Maybe you can't do the language classes as a physics undergrad? Depends on your undergrad school. Keep in mind the possibility of filling holes with non-credit classes, though that's pretty severe.
Also, many universities have scholarships that you have to apply for to be considered. Check with your school's guidance people, and the school web site. Check again when you are about to start each semester, and again when you are about to start grad school. New scholarships may have been created. An extra stack of cash is probably very welcome.
When you are about to start second year, do a review of the situation. See if things look like they are on track. You might find you need to swap degree programs. Or you might just need to switch what courses you emphasize.
Before third year you probably want to contact some profs at potential grad schools. Talk to them about whether you will be a good fit and how you could be better. Find out if they will have space for you. Maybe they already have nine people in six places and won't be able to admit you. Also, check with them what scholarships you should apply for.
And keep in mind the possibility of a "plan B." Maybe you decide in two years you are not so keen on astronomy as you thought. Or maybe still love astro but something else you learn about becomes even better. You need to live after university. So pick up classes for breadth. That philosophy class might look pretty wildly off topic, but it may look good to a scholarship committee. And it may be a great conversation starter at the school pub. That computer design class may also look like it's wildly off topic, but might get you the great summer job that pays for your first year in grad school. And so on.
