What I've learned about getting good results from knit and crochet projects

All opinions my own.

Be aware that the fibers for fine hand-knitting yarns may require special care - "real" wool will shrink and felt, acrylic will soften and loose its finish, polyester yarn can "melt" in the dryer. 

A cast-on that is not too loose and not too tight is critical to the project and can't be fixed after the project is started (you will have to rip out to fix). Same with the starting chain for a crochet project.

A bound-off edge (ending edge) that is too tight or too loose can usually be fixed by un-knitting and re-knitting the edge.

It's better to fix any mistake as soon as you see it than to keep going and try to fix it later.

Sometimes a dropped stitch in knitting can be fixed with a crochet hook, and the fix won't show after blocking.

When knitting, weaving in ends is better done after the whole item is finished - and weaving in ends along the side edges creates bulk that can interfere with seaming or edging.

When crocheting, yarn ends can usually be worked over to conceal them in the work.

Scarves knit on the diagonal tend to have a nice appearance even if knitted in simple garter stitch.

Beginning knitters should start with straight needles (not circular), size 9 10 or 11, and the purl stitch, which is easier to understand than the knit stitch because the working yarn is held to the front of the work.

Beginning knitters should also start with a smooth yarn, even a "practice" yarn, and should make some sort of simple, square project as a first effort.

In a lace knitting project, running a "safety line" through the stitches on your needles every few inches is a good idea. Making a safety line means threading a piece of scrap yarn onto a yarn needle and threading it through the stitches on your needle. This allows you to rip back to the safety line and put the stitches back on the needle if you make a mistake - because it is difficult to impossible to rip back lace knitting without a safety line, except by unknitting it stitch by stitch.

If you keep your scraps (anything over 12"), you can eventually use them for a creative project, and the colors are likely to harmonize if you tend to buy in certain color families. I use scraps for various projects.