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What does selvedge even mean? Selvedge is a corruption of "self" and "edge", that refers to an edge finished in a way that won't unravel in a piece of fabric.^[1] For knitting, this mostly just means edge stitches. 

I have struggled with this a lot. Here are the tips for how to knit better edge stitches:

1. Slow down and knit carefully.
2. Don't stretch out the stitches, and make the stitch in the previous row too lose.
3. Keep the tension consistent, don't knit the current stitch too tight either.

This article goes into some more specifics and bad habits to break with photos.

There are other "methods" for how to knit edge stitches, that tend to break into three categories.

1. Edge Stitches for Stockinette
This is the advice to knit a border that alternates knit and purl stitches around a piece of stockinette so that the work will lay flat. Most common version I've seen of this is the garter border, that is alternating rows of knit and purl.

2. Decorative Edge Stitches
There's a bunch of different varieties for this one, but the general method of these is that they achieve a neat look by slipping the first stitch of the row and working the last stitch of the row so that the stitches on either edge are stretched and are all worked in the same direction. It looks nice and avoids problems with stitches getting stretched/pulled tight.

However, if you are seaming this edge, you will end up with larger holes where it connects because of the larger/stretched stitches and a weaker seam, so this is not recommended.

3. Edge Stitches for Seaming
This is essentially just keeping track of the fact that one stitch on both sides of the seam will disappear when you seam it. A lot of patterns already factor this in when they tell you what to cast on and so forth. If you're constructing a sweater from scratch, the general suggestion I've seen is to knit or purl both sides so that the seam isn't noticeable when you use mattress stitch.