Well, that's a pretty broad request.
The broad answer to the first part (getting better marks) is that you have to figure out how you learn and what works for you in terms of performing well on examinations and assignments.
Some things to consider...
- Time In
How much time are you putting into each course now and does your understanding improve if you put more time into your studies?
- Technique
How much of your study time is spent solving problems? And how much do those problems reflect those that you encounter in examinations?
What do the students who are getting top marks do differently from you?
- Review of Background Material
Are you struggling with any material that is assumed knowledge? Often if students learn "for the exam" but don't have a great practical grasp of the material, it can come back to haunt them in later classes that build on that material. Do you spend any time systematically reviewing relevant material.
- Resources
Do you find your textbooks helpful? If not, could you approach your professors for alternatives?
Do you make use of office hours? Do you talk about problems with others in your classes?
- Distractions
How much "mental bandwidth" can you dedicate to your studies? Are you focused on any other life issues (relationships, financial stress, etc.)? What about other obligations (job, taking care of family, volunteer positions, etc.)?
- Take Care of Yourself
Make sure that you're eating healthy, exercising, getting good sleep, socializing, and getting in quality down time.
With regards to preparing yourself for a career you might want to think about...
- Co-operative, intern, summer or part time job experience. Although it certainly helps to be related to the field you want to go into, it doesn't always have to be. Sometimes experience in really basic stuff like part-time customer service can make the difference when competing for full-time entry-level positions.
- Projects that you work on in your classes that you'll be able to point to as practical examples of your skill set.
- More on the skill set - when you look into the types of jobs now that you're interested in, what kinds of skills are they looking for? What are you doing to develop those?
- Consider engineering clubs or competitive teams. Representing your school at something like a national robotics competition can be a huge plus on a resume.
- Other campus or volunteer activities. What are you doing to round yourself out?