A large part of being able to answer this question properly depends on your own specific circumstances. I can give you a few ideas to get going though.
There are thousands of foundations and trusts just set up to distribute money to good causes. Take a look through directories of these organisations and write a few page application to them with a budget.
Think about which companies might give to your cause. Do any have a vested interest in alleviating the problem you are trying to solve? Write to them with a similar letter.
Events can be cost effective way of getting started with fundraising. You may need to splash out a little up front, but a good event can give you a good return on investment.
Start with what you are good at, what people are most likely to give to and think who has significant amounts to give and it will give you a good start.