You could start by going to a website like Crowdfunder and clicking on the 'start a pitch' button. There you can get ideas for explaining and publicising your project. With this type of website, you have to state your goal clearly - what you want the money for and how much. You then have 30 or more days to raise the money (you choose the length, but there is an upper limit). The contributors pay through Paypal; the money is held for the specified length of time, and if you haven't managed to raise the target amount by the end of that time it will be returned to them.
However, crowdfunding takes many other forms. You can also raise money through organisations with more specific aims; for example, many people now carry out crowdfunding projects to raise money for a business or private venture, and there are several different ways to do that - you can see examples here.