Are you starting a business are trying to make some side cash? As these guys have been saying, building a business may be really hard to do and have a college schedule, the timing doesnt fit. However if you just want to make some extra cash mowing lawns can be a great way to do that. You can schedule your classes to be monday, wednesday, friday only, or be off by noon, or go to night classes, and still give yourself plenty of time to do this.
Just mowing lawns all you need is a mower, string trimmer, blower, hedge clippers, and some basic tools (wheel barrow, shovel, rake, etc.) and you can make some good money.
Even if you only work 2 or 3 days a week you will make more than your typical college job, atleast during the spring and summer.
Some tips, only advertise to small yards. Dont fall into the trap of wanting that big money job when you dont have the equipment for it. Its a waste of time and you'll never compete with the price or quality of the bigger companies. They however will have a hard time competing with you on small yards that only sized for a push mower. They have much more overhead and less personal service.
Do some research and find a good, affordable place to get door hangers. I used 3000 doorhangers.com when I got started. Not the best quality, and the designs are generic, but I got good results for the money. I had 30 clients that first year off about 4000 door hangers, and lots of one time jobs.
I wouldnt worry about all the legitimate stuff if you are just making some cash while in school, though lots of guys would disagree. I would to if you were starting your career with this thing. If you are starting a career business here that'd be good to know as it would change the nature of a lot of my advice. There are a lot of down the road things to consider, such as future overhead, pricing, taxes, insurance, branding, etc.