Who can become an Angel Investor?
An Angel Investor is someone who invests their own money into an un-registered startup. This is allowed through an SEC exemption for "Accredited Investors" in Reg D 506(b). To be Accredited, you need to have an Income of $200K or $300K with your spouse , or you need to have a networth of more than $1M (not including house).
To be a "good" Angel Investor, you need to understand about the structure of companies, how the team works, how the financials are being managed and many of the details of the business. You need to also understand how a portfolio of 20+ companies can be created that makes up for the risk associated with early companies that are just starting out.
A good place to start is to read the book by David Rose called "Angel Investing"
If you are not "Accredited" (yet), then you might look at Equity Crowdfunding as a pathway.