Either way, you need to do things you know about. Without knowledge and experience in your business, then you're gambling.
Here are the businesses that I've started, small and large. It's fun.
College
1) Marine Cove: Saltwater aquarium setup and maintenance, which spun off into a temporary ornamental pond construction business for my younger brother.
Medical School
1) Premed consultation service where I edited personal statements for applicants and guided them through the process. This was my first dabbling of a purely internet business.
2) A&J Electronics- this was an internet retail operation where we sold retail goods via eBay and a Yahoo! store. This started from my frustations that I couldn't find Pokemon cards locally for my nephews. I spent 8 hours searching and finally paid twice the retail price at a mall specialty store. I went to eBay, and found the same cards for the same price WITH S&H ($4.95 for each deck). My time as a grad student was worth $10/hour. I wasted $80 hours of my time and spent twice as much on a retail product. I found a way to order directly from Wizards of the Coast. I ordered 200-300 decks of cards at the retail price of $10/each. I took these decks and sold 5-10 daily on eBay. Each auction started at 1 penny, but I sold each deck for $15-$40 each. It took me less than one month to move my initial inventory. I sold Pokemon cards for 3-4 months until the craze was gone. Then I took my profits and purchased overstock electronics from major manufacturers like IBM, Sony, Canon, etc... I sold these items in a Yahoo! store and on eBay auctions. I worked 2-3 hours daily before I went to lab during my PhD. I sent invoices to my mom 3000 miles away. We built the business to ~$400,000-$500,000 sales yearly with a profit margin of about 10%. Margins for electronics are slim, and as more sellers started to compete, I folded the business after 3 years when I started ophthalmology residency, but we were in the black for all three years. I also helped several friends start their own electronic retail business. A&J Electronics taught me a lot about e-commerce, how to run a business, and marketing. It also taught me dozens of other business models as I studied other businesses on eBay. This was an invaluable experience.
Residency
1) Started my first corporation. MedRounds Publications, Inc. You can follow our progress on our website:
www.medrounds.org
We are publisher of academic materials for students, physicians, and patients. I have some big plans for this company, and many people are excited.
My advice is that if you have ideas, then write them down. Come up with a business plan. Determine your test market. Form the company then recruit investment capital. If you have your own money saved up, then you don't need to look at investors to help you with funding your young company. I recommend that you read the books in this thread:
http://forums.studentdoctor.net/showthread.php?t=194496
Rich Dad, Poor Dad by Kiyosaki
and
Own Your Own Corporation by Sutton
Each of us have a great idea that can make millions of dollars. However, many of us have boundaries and external forces that prevent us from even trying. How often do you hear others say, "that's dumb", "it's already been done", "it's too late", or "you can't do that"? My advice is to not listen to the
Chicken Littles who may surround you.
Some of these boundaries, on the other hand, exist in ourselves. You'll hear people say, "I had that idea too. IF only I had done it, then I'd make a million..." In the same breath, the individual continues to sit on the couch and watch TV.