Use "Hopefully..." to talk about something that you hope will happen. For example:
Hopefully we can finish the deal by the end of this month.
You "hope" for something that:
- might possibly happen
- isn't definitely going to happen
So, for example, you can't say "Hopefully I can be a baseball player" if you're 30 years old and not athletic. It's too unrealistic. Instead, you would say:
I wish I could be a baseball player.