What lenses should everyone have?
everyone should have a 50mm 1.8 (or 1.4) in their bag in my opinion. at $100 or less for a new 1.8 its a cheap way to get outstanding results. other than that, a decent or very nice telephoto depending on your budget. a lot of new people pick up the 55-250mm as it picks up where their 18-55mm kit lens leaves off.
however, if this is something you *really* want to do an article i read suggested that you should buy the best lens in a particular range. if you cant afford it, then you wait until you can...be it one week, one month, or one year. not sure i 100% agree with that, as there are some really nice alternatives to the best of the best lenses, but i do keep it in my mind at all times before i decide to make a purchase.
another thing to keep in mind if this is something you wish to pursue is to try and decide if at some point you will explore a full frame body. if you do then you'd want to purchase only EF lenses. EF-S lenses will not work on full frame bodies like the 5d's and the 1ds's, but EF lenses work on both.
the 50mm mentioned above (both the 1.8 and the 1.4) are EF's and will work on any canon with an EF/EF-S mount. the 55-250mm telephoto i mentioned above is an EF-S lens, and thus will need to be replaced if you decided to jump to those full frame bodies.
you probably have quite a while before you decide, however, so dont worry about it yet...just trying to provide some food for thought for ya.
