One simple thing is required for building up an email list that's valuable to you: You must be valuable to your target audience.
Generally, this means you generate amazingly useful content, that makes them better at what they are trying to do. Teach them how to be awesome at the things that matter to them.
Then you just need to give them a way to hear more from you. MailChimp has a great free plan that makes this very straight-forward.
That's the big picture. Let's talk if you are unsure how to apply this to your situation.