I make a supplemental income betting NFL and NBA. My buddy lives in Vegas and is a math genius and does NBA and I tail him, and for NFL I watch all the games. Every week. It's a lot of work. We both use situational capping a lot. All your stats and trends are public ie they're already priced into the lines. You can't look at basic stuff like points per game, etc. Or this team is 8-2 in their last 10 while the other team is 3-7. This is how people get slaughtered.
We prefer coaching tendencies, situational capping, public perception, line movement, etc.
Also, I hope you have a strong stomach. Statistical variance is a bitch. You will go 70-30 and make a lot of money and follow it up by going 40-60. You have to have good money management and not be emotionally attached. Whatever money you have for betting you should only use 1% per bet, so if you have $10,000 only bet $100 per wager.
Sports are always evolving, too so you have to keep up with them. It's like poker and trading - you need hard work and great money management while keeping your emotions out of it. You're looking to repeat an investment into a profitable situation over and over again long-term. Short term variance will always be nuts.
I've made over 15,000 bets the past 6 years and have a 56% win-rate. People see that and don't realize how good it is. That's against an average line of - 110, and making Constantly betting large favorites is a trap that will never be profitable long-term. Same thing with parlays. Stay away from them.
You're grinding out units over a slightly better than break even bet rate. The more money you have behind you the more these few units turn into from a profit standpoint.