When I first started to crochet, I didn't have a clue as to what gauge or swatching was. To me, a gauge was the thing you used to check your tire pressure. Nor did it really matter all that much, as I started with items like scarves that didn't require precise measurements.
Then I came across a pattern I wanted to make that listed the hook size as "whatever size is needed to obtain gauge." Huh? How exactly was I supposed to know that? Thankfully, the pattern also had instructions on how to make a swatch.
A swatch is a small piece of crochet fabric you make using the yarn and pattern stitch of the item you want to make. It should be large enough to get a good measurement of how many stitches are needed to make an inch across and up. Don't make the same mistake I did at first of trying to make a 1" x 1" square! For some reason, your stitches even out the larger the piece gets, so try to make the swatch at least 4" x 4". It seems like a frustrating waste of time that is preventing you from making what you want, but it is necessary to ensure it your finished item will be the correct size.
There are many reasons that gauge can be off, from crocheting too tightly or too loosely to the type of yarn being used. There are just as many ways, if not more, to correct it. You can start by making a swatch using the suggested hook size, either in the pattern or on the yarn label. Make swatches of the same yarn with different hooks until you obtain the gauge you are looking for. Stitch height is a bit more difficult to correct, but there are many tutorials on the web on how to do it.
Here's a tip that I learned yesterday - save your swatches, every single one. Mark it with the type of yarn, hook size, and stitch pattern you used. That way you'll have something to reference in the future, therefore cutting down the time required to perfect your gauge.