Google finds the ads to put on your site automatically, according to the topic of your site.
You can image, an insurance ad is much expensive than a fast food ad. So if the topic of your site is "Insurance", then Google will find ads relevant to insurance to display on your site. You can make more money from the ads than a fast food relevant site. (This is not absolute, it is general)

Then you should try to adjust the style of your Adsense ads to fix the design of your website. The key is make the Adsense ads do not like ads, let them be friendly to visitors. Than visitors will be more likely to click your ads.

Just research keyword phrases on the Google adwords keyword research tool. You will get an idea of what the cost of different keywords related to your site cost advertisers. But you also want to rank for those keywords if possible with the highest cost per click.... Find a high cost per click phrases thats not too competitive. Which is probably gonna be a pain especially if your in a competitive niche.

Then optimize your sites content with those keyword phrases, build backlinks, and start driving that traffic to your website, or blog. Google will place relevant ads. If your a blogger use the adsense optimizer plugin for Wordpress it is free and they place the ads in high click locations.