Mee-ow to you all. It is I, Mimi.
The first and most often used is to crochet two (or more) stitches together and it is written in patterns as tog, eg 2dctog.
To do this we start the first stitch but we do not finish it and instead keep the last two loops on the hook like this:
Then we start a new stitch in the next loop along on the previous row like this:
Now if you look at the stitch we have just made closely, you will see that even though there are two stitches below there is only one loop, therefor only one stitch will be crocheted in the next row.
So that is crocheting stitches together.
The other way to decrease is to leave out or skip (sk) a stitch entirely like this:
This may seem easier, but this technique is not usually used because it leaves holes.
However, in certain pattern styles, such as lace or the zig zag of Chevron where the missed stitch(es) creates a V pattern, this second decreasing stitch approach is used.