Be curious, critical, serious, and considerate!
Attend classes, read books, and make the most of the extra-curricular activities in and outside university.
Start looking for internships in your first year.
If you have the opportunity, go a year abroad to another university.
In particular, at university:
Select the modules you are really interested in.
Start thinking early on about your own dissertation (even if you are in your first year). This is your opportunity to create your own work from finding a research question to making your argument.
Be aware that, in class, handwriting is more efficient than typing on the computer: http://www.sciencedaily.com/releases/2014/04/140424102837.htm.
Use seminars as much as possible to express your views, and develop your skills in organisation and criticism.