Well there are a few different ways, it depends on how exactly you want to get involved.
1. Most people choose to give money or support with materials which can be helpful.
2. Work in a homeless shelter or other organization that aids the people who live on the street. There are many organizations everywhere.
3. Get out WITH OTHER people and sit down, form relationships with the people of the street and work to show them that they are in fact worth something, that someone does care about them, and that they can care about themselves.

When you go out be sure to not go out with the thought that you are going to "fix" the problems. First you must understand the issues with EACH individual. Each person can have different reasons for being where they are in the life. The most important thing I feel is that you show the people of the street that you care and that they are in fact worth something. The best ways to help take time and effort, there is no automatic fix for homelessness. First you must understand, then you can work with, but ultimately the people have to make the decision to get off the streets. Sometimes just sitting down to talk with someone and treating them like a human being can be enough, other times you may have to come back and talk 100 times before they realized that they deserve a degree of dignity they are not affording themselves. When we begin to care for others they begin to care for themselves, and that is one of the many problems that keep the homeless on the street, low self-worth. I am excited to hear you want to get involved and I wish you the best of luck in any endeavors that you may encounter in the world of the homeless. Just remember that love is the most powerful tool you have, each person merely wants to be treated like a human. I hope everything goes well.
A little advice: remember they are people and there are many things you can talk about: weather is very relevant to the homeless, firewood as well. NEVER GO ALONE!!!! Take food! Food allows you to make a connection. We usually take a sandwich and a piece of fruit to approach or what we call "getting your foot in the door" ironically they don't have a door. haha When you do this you can offer them food and with that spark a conversation that could create a relationship. If you can create strong relationships then you can really make a difference. You often move them emotionally and physically and sometimes if you really listen you can hear some of the most profound life lessons and be moved yourself. Sometimes you will gain more from the experience than they do. Some will not be accepting so continue on and don't get discouraged. DO NOT GO TO PLACES THAT ARE SUSPICIOUS or OBVIOUSLY ILLEGAL. It can be scary at times and rewarding. I cannot stress enough to go out WITH others and wherever you go think of it as being a guest in someone else's house. Respect and love and in the end many times if you get yourself familiar with various organizations and/or services that you can recommend if they ask. Don't pressure in a religious way either. If faith emerges in the conversation, which it will very frequently because many of them only have their faith left, go with it and talk with them, but if it does not come up do not bring it up because many of the people have issues with God, faith, and/or churches. Again! I hope everything goes well! and if you need any other advice be sure to get in touch.
Good Luck!

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Live the Life of Love
We are meant to be.
I am not "most people"