There's so many different kind of traffic source types: Push, Pop, Native, Social, In-App, Banners, Solo Ads etc...
IMHO, the best traffic source for newbies would be PPC because you're dealing with high intent traffic, and you can target very precisely who will see your ads.
- It doesn't require any initial deposit - you only pay per click.
- Traffic is very targeted and hence slower, so you don't burn your budget in an hour
- Affiliate networks will love the traffic quality you're sending them
- Once you get a campaign profitable, it usually lasts way longer
I would recommend starting with Bing Ads over Google Ads. Much more traffic on Google, but Bing is still sending quality and good amount of traffic in many verticals, and they're way less strict than Google.