In order to earn from ad networks, you need engagements on the ads. If you have low traffic, you will have no engagements on your ads. No engagements means no revenue. Placing ads on the website does not mean anything if you do not have good number of visitors. Therefore, you should never hurry to had ads on your website. It is very difficult to build organic traffic, it might take months, even years. In that case, you might want to use ad and earn some money from your website. In that case, you can use any ad networks that you were approved previously. For example, if you already have Adsense, you ca use adsense even if you might not have good traffic. In case you want to apply for ad networks for your low traffic website, you might be rejected. If you still want to have ads, try infolinks, vigilinks, chitika, bidvertiser, even adsense.