Monetizing a website can take on many flavors.
The first question is whether to monetize or not. If you have a website for family and friends or for a closed entity of some type, monetizing may not be desirable. If you have a website where a lot of people visit every day because you sell something, have great information or have free stuff for the to download, then monetizing may be a real good thing. This site is actually getting over 200 hits per day and monetizing makes sense. It is not large enough for google adsense as yet, but there are other ways.
Your Site is like Real Estate
Remember that a website is like Real Estate! Your pages are your lots and the space is valuable. You paid money to have the pages placed on the server and more money to have the pages built. The space has value if the old rule of Location, Location, Location is followed. If you have traffic of 2000 a day or more, advertisers will be easy to find and some will come to you. You can charge for the space like a 125x125 pixel add for $xx.xx number of dollars per month. Location is what type and what niche you fit into. It seems, that sites with a store(ecommerce) is a big draw. It seems like membership type sites are also a big draw. A forum and/or blog helps out as well. Attractiveness also helps as does good content and SEO.
Assuming you have 400 or more visitors a day, then adsense may be an option because adsense works on a monthly basis. If you don't hit a certain number of clicks and earn a certain amount, then you lose it and start over. It can be very good if you get enough clicks in a month. Be careful when signing for adsense, since they make the most money off of adwords and will push you to do that. Adwords does make sense if you have the budget to sustain it. It is like having you site always at the top of the google search engine.
The affiliates programs are everywhere, but care is needed to get the right ones for your site. One way is to visit sites and look at the bottom to see if they have an affilliate program. The other way is to join an affilliates company where they have many programs to offer you like Linksharewhere you join for free and ask the advertisers for the banners or text to add to your pages. You get commissions off either sales, memberships or clicks. If you noticed, this site is monetized and I am using Linkshare for that. There are several others as well. It is a good idea to just use one affiliate's company and not have several with banners from each. If you are concerned about how much, it is not much for each, but it can add up and in most cases is enough to pay for the hosting and building of a site over time. There are instances where one could make some serious money this way.
There are rules for each company and each advertiser as well. Usually, that means keep it clean on the site and do NOT self promote the ads. Just let people click if they want to.
You can do banner swapping with several companies out there. There are some pitfalls, but is a good source of extra visitors. You would need to create at least one banner and dowload your choices of other peoples banners to place on your own site.
How it generally works is, every time your page is opened that has the banner on it, a message is sent and your banner gets placed in rotation to be shown on other sites. This means that the more visitors you have, the more your banner(r) are shown on other sites and that will get even more traffic. I have not placed these on this site as yet, but when this is completed, I intend to put one on each page somewhere. With 140 people a day coming here and clicking multiple pages, I can get fairly good exposure on the internet. You can create a banner that changes every 1-2-3 or 4 seconds by making an animated .gif file which most will accept. Like these on another site I created for application allstars: http://applicationallstars.com/blog/index.php/new-allstar-applications-and-application Those banners are everywhere now because application allstars and allstar application has a traffic count over 500 per day on the wholle.
You will need to know your monthly traffic count when signing up for your monetizing plan. You can find that in several places. If you have access to cpanel of your site, it may be the quickest to get the counts they generall have something like "stats" in there that you can open. You can use my favorite Site Checkupfrom any browser where mysite.com would be your website address without the http://. If you have signed up for google analytics, you can get the counts there. There are more places and ways, but they should all give the same numbers. Usually by the daily count, so you need to do the math and multiply by 30 for a monthly count.