Do a search for list building ideas. That will give you a start and I spotted an article from Hubspot that lists 29 suggestions that didn't look too bad. The main thing to keep in mind is to not jump into a bunch of techniques all at once. You'll get bogged down with information overload if you do. The best way to proceed is to select one or two at a time, learn it or them well, and then add more methods over time as you get comfortable with using the previous ones.