Which skill would help me become a good mechanical engineer?

 There are many things that you can actually do to improve your profile :

1. Projects : Start taking up some project obviously related to your field! To begin with start with mini projects. Projects play a vital role in your resume. So, don't think that this is too early for you to begin with projects!
2. CAD/CAE : Start learning a CAD software. For example you can start with CATIA, PRO-E, SOLIDWORKS. Learn and master atleast one of them! You would also come to know that whether you have any penchant towards the CAD field in your near future. After CAD, Start leaning CAE softwares like ANSYS, HYPERMESH.
3. Online Courses : Start learning online from websites likehttp://coursera.com , They have courses related to Mechanical Engineering. Once you are successfully done with the courses you get an online certificate which is really helpful.
4. German language : When you say you want your career in Automobiles, the first thing that comes up is Germany. Start learning a language and for your interest German suits the best. This will surely make your profile stand out.
5. Academics: Don't let your academics go low. They are really important for the job as well as the masters. Remember, high scores will always help.
6. Internships: Start finding internships in industries right from now. And preferably in Automobile Industry, if not then whatever comes your way. Grab the opportunity of doing internships, it really helps!

Plan your time really well and I'm sure that you will be able to do all of this during the years of your engineering tenure!

Hope I was able to answer your question to some extent.

All the best friend! I wish you a bright future :)