If you're interested in either of those you need to understand that it takes a lot of time and energy to build up a following, you'll not be able to instantly sell hundreds of t-shirts per day or get many students for tutoring purposes. These are long-term opportunities that can even become your main source of income depending on how hard you're willing to work.

Selling shirts is probably the most straight-forward option, you need to find a site like http://www.teespring.com where you can design your own clothes. Choosing a design that is modern and will appeal to a big audience is the safest way of gaining traction, right now Christmas sweaters are selling like hot cakes.
After you finish your creation you'll probably need to launch a campaign, once you reach the number of shirts you're expecting the site will print and ship them. Many people have taken advantage of the newer shirt sites since the old ones are over-saturated by veterans.
Tutoring people is another good opportunity that not many people are aware of. If you have a skill you can teach like programming, maths, foreign languages etc. then you're in luck. You just need to find a site that allows you to be a tutor, there are many online. Usually you set your hourly rate which can even be ~$40-50/hour if you're good at the subject you're trying to teach, I know people that quit their offline job in order to work full time as a teacher online. The niche is one of the most interesting and many students are looking to learn currently, especially with exams coming up.
The third and last one is flipping items on eBay or Craigslist. You may ask what "flipping" is and it's not that difficult to understand, the term basically refers to buying things for cheap/below the market price and selling them online for profit. Yard sales usually have items like these laying around, that's how many "flippers" earn their money. A good idea would be to find a field you're knowledgeable about and focus on finding items regarding it, as long as you know the true value of an item you can find many deals. One of the ways I tried to make money was to buy discounted video games from stores, many at around $1-$2 and flip them online for $5-$10.
I managed to make some profit but with my real-life job and studies it was quite hard to keep it going, I had to forfeit the "business" after a while due to lack of time.






