There are only four basic stitches to master: running stitch, oversewing, herringbone and blanket stitch. Running stitch (probably the easiest to start with) and oversewing can be used to make clothing; the other two are for decorative edging.
Machine stitching (only plain stitch, not zig zag or overlocking stitch) may be used on internal seams only. Visible sewing (e.g. hems) should be hand-sewn; if you do machine-sew it, please sew over by hand (so it’s probably quicker to do the job properly in the first place!).
Note that hand-sewn stitches should be small and neat. Some surviving needlework from our period is so perfect that modern machine stitches are put to shame. Aim to make stitches at most 5mm apart (and often less). This may seem a nuisance, but large stitches have a habit of catching on rough edges (splinters, weapons, mail) and breaking, leading to chunks of your sewing unravelling. Please don’t think that you’re saving time by putting in centimetre-spaced stitches – if it doesn’t break and need to be redone, you will be asked to re-sew it if you come for a kit check!
You may find it helpful to double-back on yourself as you sew. Having gone 10-15 cm forwards, if you then put a couple of stitches the other way, you’ll make it much harder for a whole seam to unravel. It’s not a great deal more effort, and very helpful. After all, a stitch in time…
As a rule, the thread you use should be the same as the fabric you are sewing – i.e. linen/cotton thread when sewing linen, and wool thread when sewing wool. There may be exceptions (e.g. if you’re using a linen lining on a wool hood or cloak), so use common sense.
Worked from left to right. Several stitches can be picked up on the needle at once before pulling the needle through. Use this to join two pieces of fabric.
Worked diagonally from left to right. Use this to reinforce running stitch, and to secure hems and seams.
Started at the top of the row. Work from left to right alternately taking a stitch at the top and a stitch at the bottom. Use this to reinforce running stitch, and to secure hems and seams. With the X pattern on the outside it can be both structural and decorative - but can be prone to snagging on things.
Worked from left to right. The needle is brought up vertically and brought out of the fabric with the thread tucked under the needle. Use this to secure hems. In a contrasting colour, can be a simple and cheap form of decoration.