The two best ad networks for new general audience sites are Fastclick and Casale Media. Both networks have relatively low entrance requirements and aren't as strict about subject matter as some of the other networks. If your site fits well into a high profit niche (generally, anything other than entertainment (books, movies, games, sports, tv, etc), then try Burstmedia or TribalFusion. They have slightly better rates and can sometimes get you targeted campaigns which pay a premium over general audience advertisements.
The above mentioned ad networks all offer both CPM & CPC ads with the exception of TribalFusion which is CPM only. They also all offer a variety of ad types, including leaderboards (large banner), standard banners, popunders (ads that popup underneath the current window), popups (ads that popup on top of the current window), skyscrapers (tall narrow ads), and boxes (square or rectangular ads often found imbedded in articles).
For CPC ads the best network is Google Adsense. Google Adsense automatically analyzes the content on each page and then serves related ads from Google's library of hundreds of thousands of Adwords ads normally found on the side of search results. However, as I discussed before, if your site visitors do not tend to click often or if your content isn't attractive to advertisers Google Adsense will not do too well for you. Adsense also has a few competitors, such as SearchFeed or Revenue Pilot, if you do not get accepted by Google.
You do not need to pick one ad network, try to join them all and then use the ones who do best for you. You can use more than one ad network at a time. No ad network can fill all of your inventory (not even Google) so if you send 1000 impressions to an ad network they might only serve 500 ads. The other 500 ads are called defaults, and you get to specify to the ad network what they serve for those ads. So typically what publishers do is form a chain of ad networks and send defaults from one to another. So for instance your primary network (best paying) may be Tribal Fusion, but Tribal Fusion can only fill 50% of your inventory, so you tell Tribal Fusion to send it's defaults to Fastclick. Fastclick then fills 50% of your remaining inventory, so you tell Fastclick to send it's defaults to Casale. Casale then fills 50% of your remaining inventory from Fastclick. So instead of only serving paying ads 50% of the time had you just used one network, you serve them 82.5% of the time. Just in case I'm not clear, or that you take me too literally, 50% fill is just a percentage I chose for this example. In reality ad network fill rates can fluctuate between 80 and 20%.
For CPM advertisements networks have rules about ad placement. You may think "Okay, if I make $1 CPM for each banner I'll just put 10 on the page." Fortunately for your site's visitors such ad stacking is not allowed. Typically you can put only one of each ad type on a page, though some networks allow both a top and bottom banner paying at different rates. Ad networks also usually require that you put ads above the fold, that is where a user on a 800x600 resolution would not have to scroll down to see the ad.
For CPC ads it is in your best interest to present the ads in such a way as to give them the most attention. For instance with Google Adsense most people find the highest paying ad size is the box like what is used on Website Publisher articles. The reason is that this ad type is most often situated right in the text of an article so that users have to see it. In contrast a top banner is often glazed over by users. You can also experiment with color schemes to see what generates the most clicks.