I'd like to make a sweater from a pattern in a book I have, but I'm a bit confused on how to measure the gauge once I've knit it. This will be the first time I've used a gauge.

The book says "17 sts and 32 rows (16 ridges) = 4"/10cm in garter st using size 7 (4.5mm) needles."

So I cast on 17 stitches and knit 32 rows, right?

Since I'm using a long tail cast on, would that count as the first row?

When I finish the gauge, do I take it off the needle or leave it on? If I take it off, do I bind off? If I bind off, is that counted as a row?

Is the gauge supposed to be a 4" X 4" square?

Don't concern yourself with the CO row. It's not a consideration on the swatch.

It's not even considered a row on knitted items.

some people choose to leave it on the needles then when they have measured they cn take it of and unravel it to use the yarn, other people prefere to bind off and keep the swatch for their records. IT is up to the knitter.

But as has been said cast on around 20 stitches or so and knit for a good few inches in garter stitch (since that is what it calls for this time) when you have a square, pick a patch in the middle of it and count the number of stitches and rows in an inch. multipply this by four to get your gauge. If your swatch is big enough then you can count the number of stitches in 4 inches, but make sure you can stay away from the edge stitches as these can often be a little different.

so if you count 4 stitches and 8 rows (4 ridges) in 1 inch then your gauge would be 16 stitches and 32 rows (16 ridges) in a 4X4 square and your guage would be almost spot on. maybe jsut keep an eye on tension for that extra stitch.

It's better to be able to count the number of sts in a full 4", so it's good to use at least 20, 25 would be better in this case. Sometimes if you only count how many in an inch, you may have a partial stitch.

Here are some useful thoughts about calculating gauges while knitting or crocheting:

http://www.smart-knit-crocheting.com/beginner-crochet.html