So, I have a few suggestions:
1. Pray. Ask God to guide you and put on your heart where to go and how to help. My church almost got a building in a ghetto to relocate to a permanent place, they were renting prior, and we were going there weekly to evangelize. It was awesome, we prayed for people, it was great, but God didn't want us there permanently.
2. Go to the streets and help the homeless. I lived next to a university which had a homeless issue down the road. So, my friends and I would go out and talk with them. We'd humanize them, show they mattered and people cared. We'd hear their stories, we'd pray for them, and give them food. Some friends would take a cooler full of food and sandwiches to give them then pray for them. Other friends and I would invite them to eat. No joke, we once put a homeless lady on a bus back home. She couldn't have been more happy.
Now, I know you're saying you want something large scale and that'd be nice, trust me, it would, and the ambition is great, but not everything needs to start up large scale. Sometimes, building up is the way. As you hit the streets, as you do what you can, a picture may start to emerge of how these people need help in ways that they currently aren't being provided for and the answers will come to you.
Also, if you do the above, I strongly suggest trying "treasure hunting." Before you go out, pray, ask God to tell you what to look for, and usually he'll give you a sign. Keep doing it. It'll pay off. It really will.