See what infrastructure (city boards, committees, etc) are already in place and you can work through those or you can organize from scratch.
I was very involved in the pioneering days of the gentrification/redevelopment of a certain south Florida area known to be gay and we went from drug rehab on a main corner, crack deals everywhere to getting written up on the front page of the NYT, Saturday edition and below the fold, but still, pretty cool.
Also a good friend at the time was very involved with a similar project in the county to the north and helped turn that from a very dumpy area to a very successful area.
If you are interested in improving the area around your residence or a nearby downtown, start going to city hall meetings. Try to get on one of the boards whether rezoning or code enforcement or bd of adjustments, or just attend their meetings and speak out on issues. Get yourself known to the community so that you have some say in decision making and so you can get on those aforementioned boards.
See what your city has in place already. You might even find some sort of matching grant program whereby your city will match funds raised by your neighborhood. Some of those funds might match an equivilent of volunteer work such as the city providing materials if the neighbors provide the labor.
You might have a community redevelopment agency with access to Federal funds. A Chamber of Commerce might have some joint venture program with a city but probably you want to start off at your town hall. Go talk to one of your city planners, tell them what you want to accomplish and ask for suggestions as they will know what is in place or what you would need to first establish.