Join study groups with like-minded people. Attend classes regularly and get into the habit of taking detailed notes during classes when the professors speak. Note taking is a good skill that can come handy at any point in life, be it your academic stint or your professional career. Work on all your assignments by yourself and only consult other students/TAs/professor only after you have given your best effort. Make Google and YouTube your best friends; understanding the concept is very important; also developing the ability to explain the concept confidently and in layman's language to another person is even more important. Be part of organizations or committees that are relevant for your STEM field. For example, if your major is Mechanical Engineering, you should be part of the Mechanical Engineers Association, Sales Engineering Club, etc. Be dedicated to these clubs and try to get into leadership roles in these committees. Networking is very important; try to network with as many people as possible, even though they are from a different major, try to get a broad overview of what their major is like. Work on improving your soft skills: presentation skills, communication skills, etc. Talking is important, but listening is even more important. Listen first, talk next.
Then, focus on getting into a good summer internship program and work diligently throughout your internship by learning and networking as much as possible. If you do well here, you have successfully taken the first step towards your professional career, who knows, they might offer you a full-time job at their company already.