There are an infinite number of ways to do this. Remember that you get paid for adding value to people's lives. Paid webinars, monthly newsletters, selling or reselling via ebay/etsy, ebooks (especially via Amazon/Kindle), digital products (check out Clickbank) as a creator or affiliate. Look for something you're interested in so that your interest won't wane. If you're only in it for the money, it won't last long and you won't have the drive to make it high quality. Check out Pat Flynn's site Smart Passive Income for ideas and resources.
A key for any business is to leverage your time and create recurring revenue. Freelancing tends to be another job (dollars for hours). Creating something once that can be sold an infinite number of times (all digital products) is the key to growing your income.
Developing a network of supportive people who can help you build your idea and can later help you promote your products/services is also critical. Don't think you can do it all yourself. Leverage (and support!) your connections.
It usually takes time to develop the infrastructure and following/tribe/audience/customers. Don't give up too soon. Eighteen months is a good benchmark. After that, if you've put in the work, the business will start to take off. Don't expect "fast cash" before that though. Be willing to put in the hard work, delivering value all along the way (usually for little or no money in the beginning to develop a following who will buy from you later).