how do i pick macro lense for my canon slr camera?

If it were my money it would go for the Canon EF 100mm f2.8 Macro USM.

The 100mm focal length has been my choice of macro lens for years. I like being farther away from bees LOL!

2nd. choice would be the Canon EF-S 60mm f2.8 Macro USM.

your choices are pretty easy either go Canon or Sigma .
start by going to local camera shop – bring your camera and try them at the store.
If you have a canon rebel XTI the stock lens is pretty good if you give a good try – it goes down to 17mm .

lens requirement will depend on what you are taking pics of and how many – lens do not come cheap

We really need more information to answer. Or I guess the answer should be- what do you want to photograph?

If you want really high magnification, the best way to do it is Canon’s 65mm MP-E lens, but that’s $ 800 and it can’t even focus OUT past 1:1, so you couldn’t even get most flowers into the frame- you couldn’t shoot a rose with it.

A cheap way to get fairly high quality macros covering a large range of sizes including really high magnification (5:1 or greater) is to buy a macro bellows, a reversing ring, and whatever step-up or step-down rings you need to put your lenses on the reversing ring. This has the advantages of being cheap (everything for $ 60 or so getting cheap Chinese stuff off eBay), versatile, and pretty high quality, but it had the disadvantage of being a relative pain to lug around, set up, and use.

Also doing it on the cheap, there’s extension tubes, which can be used with or without reversing rings.

Then there’s several Canon macro lenses that don’t macro anywhere near as much as MP-E, and there’s a lot of third party macro lenses. Few of these will even take you in to 1:1, if that, but then maybe that’s all you need.

Another good way to do macro fairly conveniently, without getting into bellows and reversing and all that, and still cheaply, is to buy a high-quality old manual focus lens and an adapter. Very high quality Canon FD mount macro lenses can be picked up cheaply on eBay. Generally you don’t want to adapt FD lenses to an EOS camera, because EOS uses a shorter focal length than FD, so to focus to infinity, you need a lensed adapter, which reduces your image quality. But with Macro, focusing to infinity is a moot point, so you can buy a cheap FD-EOS adapter, take out the lens in the adapter, and mount a high quality old Canon FD macro lens on it. The unlensed adapter will actually increase your magnification.

There are many ways to go, offering many combinations of quality, price, magnification, and convenience. The easiest thing is to just buy some random Canon Macro lens, but if you want to make an informed decision, there’s a whole lot to know. And the first thing is to start with what sort of stuff you want to shoot.