Scrum doesn't really cover the strategic part of planning. In terms of building a vision and a general roadmap Scrum itself won't help you much. However, at this level planning doesn't really differ from what you know, or should know, from other methods.
So unless you know, in general, what you want to build don't bother with going deep into any method.
Estimation is a whole different thing. Once you you have a vision and a general plan you definitely want to be able to say what you're going to build in what time. Here is an excellent two-part video of Mike Cohn talking about agile estimation:
It should dispel your doubts.
You can also check Mike Cohn's book dedicated completely to this subject (thanks Jesse for the comment).