Beginning knitters can go a long way knitting rectangles and squares, but there comes a point when shaping your knitting will be necessary, whether you're knitting a hat, a sweater, or a sock.
Most knitting patterns specify what kind of increase or decrease they want you to use, but what if they don't? Or what if you're designing something for yourself and are unsure which way to go?
Check out the Published Patterns
Generally speaking, published patterns will tell you what kind of increase (and, more often, what kind of decrease) is best for that particular pattern. A lace project, for instance, will likely use yarn overs for increases, while a sock usually has balanced increases (a knit 2 together on one side of the foot and a slip, slip, knit on the other, for instance).
But if your pattern just says "increase 5 stitches on the next row" or "decrease 10 times evenly across round" sometimes it can be difficult to decide what kind of increase or decrease to use. Here are some tips to help you decide.
Read the Rest of the Pattern
Sometimes one direction in a pattern won't specify the kind of increase or decrease to use, but later in the pattern, you'll find a place where a particular one is called for. Unless there's a clear reason not to use the same (for instance, you see yarn overs later in the pattern but the particular section you're working on isn't lacy) it's a good guess that the same increase or decrease can be used where you are.
Think About Shaping
Usually, the typical increases (knit in the front and back and make 1) are considered relatively interchangeable, though they do have a different look. That's not so with decreases, which slant depending on which stitch the needle enters first.
Knit 2 together (and it's wrong-side sister, purl 2 together) slant to the right, while ssk and her sis ssp slant to the left. Sometimes it doesn't make a difference which direction the decreases slant (like on the top of a hat, unless the decreases are meant to be decorative) so you can just pick one and decrease that way throughout.
If on the other hand, you're doing shaping that will stand out in the finished project, you may want to consider working a left-slanting decrease at the beginning of the row and a right-slanting decrease at the end, so the decreases point into your work. Or, if you like, do the opposite for a different look.
Do What You Like
If it doesn't look like the manner of increasing or decreasing you choose will make a big difference in the way your finished project looks, just use whatever increase or decrease you are most comfortable with. I tend to default to knit in the front and back and knit 2 together, but you could just as easily use make 1 and ssk if you prefer them.
The most important thing to remember when it comes to increasing and decreasing is to be consistent. If you work knit 2 together on the first decrease round of your hat or mitten, you need to keep working the same decreases on subsequent rounds or your finished project could come out pretty funky looking.