When More is Less and Less is More
The first thing to do if you are getting too many results, especially ones unrelated to what you want, is to use more words. Why search for car when what you really want is a yellow dodge pick up truck? More words in the search box will usually produce fewer results. But the interesting bit is figuring out how to make sure that the smaller number of results contain what you're after.

What I Mean Is...
The trick to getting the right results is to use the correct words in the search box. Try to imagine how the web page author would have described what you're looking for and then do the same. You may have to give it a few goes before you hit the jackpot.

Phrase it Well
In addition to using multiple words, you can also use a specific phrase. If you put the phrase in inverted commas such as "dodge pick up truck" then the search engine should only return web pages that contain that exact phrase. Be careful though because if you try the phrase "yellow dodge pick up truck" the search engine will not return web pages that say "the dodge pick up truck was yellow" as it isn't an exact match.

Yes and No
Another way to restrict the number of web pages that are returned is to use the AND (+) and NO (-) operators. If you type: +yellow +dodge +pick +up +truck into a search box you should only get web pages that contain all 5 words although they may appear anywhere and in any order. This search on a search engine called Google currently returns 57,000 web pages. (Compare that number to a measly result of just 2 web pages for the same words inside inverted commas "yellow dodge pick up truck".)

You can also do it the other way around by using the NO operator (-) before words that you do not want included. For example if you are looking for any type of MG Rover except a yellow one then you would use:
    +mg +rover -yellow
This allows the words to be anywhere on the webpage but won't return results with the word yellow.

You can also combine phrase searching with the AND and NO operators. For example: "mg rover" -yellow
This requires the words to be in that exact order which restricts the search to fewer results. By adding -yellow it further restricts it by not allowing any results with the word yellow.
Or have fun and use all three: "mg" +rover -yellow

Remember not to leave spaces between the +, - or " " as spaces will mess up the operators and you'll end up with all the results you didn't want.

More Advanced search options
Some search engines have other advanced search features that allow you to limit your search to a specific website or a specific type of domain (.com only or .edu only). These can usually be found on the Advanced Search page which is normally linked to from the homepage.