The biggest thing that I, as a parent and teacher, think is most important in reading is knowing the sounds that each of the letters make. And yes it can get very confusing at times. But there are a few simple rules that can make it easier. So here are some of the letter sound and rules that govern them.
Short a, e, i, o, u All have their basic sounds
Short a --
Yes there are some other rules like ou, oo, aw, ow , bossy "r" and others but I want to leave those to another time.
The thing that has helped my son a lot, with his reading ability, is having little words like the ones above on flash cards and having him go through them. I had him just start out with 4 different groups each vowel has at least 4 of its own groups. Once he has mastered that group then grab a new one and start incorporating it into the circulation. These first group of words needs to be rhyming words, it really does make a difference.
Here is a couple groups from each short vowel.
-at: cat, mat, sat, pat, rat, flat, hat
-an: man, can, van, ran, fan, pan, plan, tan
-it: hit, sit, fit, pit, kit, bit
-ig: big, fig, wig, dig, pig, twig
-ot: hot, pot, lot, not, got, dot, spot
-op: top, pop, cop, hop, mop, stop, drop, chop
-et: pet, net, set, met, wet, let, jet
-ed: red, led, bed, fed, shed, sled
-ut: cut, nut, but, hut, shut
-ug: bug, hug, dug, rug, jug, slug, mug, plug
I hope this helps with your child with getting better at reading. This is not the first step that you need to take. Learning the letters and sounds is first. But once they know the names and sounds of letters this will help a lot.