Don't re-invent the wheel!
You're going to have a tough time attempting to help stray animals by yourself and, as you pointed out, especially as a young adult. Opening a shelter is a massive commitment, not just to keep the animals but also to understand what is best for each individual case, pay for vet fees etc. Once you take on this commitment you have to follow through and ensure the animals welfare is paramount. Your passion for these animals is laudable so use that to your advantage and join other people who are trying to accomplish the same thing as you.
In my quick search around the web (not knowing your exact locality) it appears there are multiple charities that help to tackle street animals and provide education to stop more animals having to live on the street. You may wish to research some of these and (with the appropriate level of care and supervision / permission from guardians etc) start helping out at a local centre.
You say that you don't have the money to put into a shelter, that's fine! Don't worry about that. What you could do is to help get more funds for the charities that can do the work. Obviously, I don't know your whole situation but I'm sure there must be something you can do to raise money for a charity. Even the smallest amount can help out so don't be disheartened! Perhaps others at your school might be interested in helping to fundraise. It's always easier to do these things with other people.
Research / Awareness
As others have pointed out, the problem of stray animals may be mitigated by spaying and neutering but you only find these things out by reading more about the problem. There may be simpler ways to help these animals than you think. Even if a charity cannot help you directly, most of them will be able to give you some advice on what you could do. They are the experts, after all.
Your intentions are great and I applaud you for taking an interest in animals which need someone to help them. If you really want to help them out, make sure you investigate legitimate ways that you can help the animals first before attempting to start a shelter.
Some charities I found
Make sure you thoroughly research the charities first and get consent from appropriate guardians before doing anything. I know nothing about these charities, they are just linked on Google.