The easiest way to monetize a blog is through Adsense. If your blog meets Adsense guidelines and you get approved all you have to do is place the adsense code on your blog and with enough traffic you can earn money when someone clicks your advertisements.
The amount of money you can earn through your blog and Adsense will depend on how much traffic your blog gets and how much the advertisements pay per click and how many clicks you receive per day or month on the adsense ads.
Never click your own ads though as that will get your account banned.
Earning with adsense will take time though as it's not an overnight get rich quick scheme.