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Unfortunately, 'entry level' jobs are no longer for people without experience. You need to get some experience as a full-time employee under your belt before you're ready to apply for business development manager positions. Seek out limited, part-time positions, internships, and apprenticeships first. After getting a few real world applications of your degree on your resume, you should be ready for a full-time job. These can be found on job websites. You should also apply with any companies you with which you've interned in the past. You already have a relationship with them - just make sure it's a good one! Since it is less common and expected to spend your entire career in one company in the 21st century, moving up the ladder may mean getting on a different one. Use the connections you have made a long the way to look around for job opportunities along the way. But don't jump ship a few months after getting that first job. Establish some longevity to demonstrate your loyalty to businesses you want to apply to work for one day. The business world likes its applicants to mirror formality and give flashy presentation in interviews. It's time to break out your suit, even if you are unsure if that will be everyday attire in the job. For more tips on acing a business job interview see this guide.
Look for internships and part-time jobs. Apply for entry-level jobs. Look around for promotion opportunities. Practice your presentation in job interviews.