To do a charge move, hold the first direction for about 2 in game seconds first.

So let's say you use Guile's Sonic Boom, which is back, forward, punch.  You then hold back for about 2 seconds, then press forward + punch.

For charge moves that are super attacks, same thing applies.  Balrog (the boxer) has a super that's back, forward, back, forward + punch.  So what you do is hold back for two seconds, then press forward, back, forward + punch.

You can charge pretty much anytime.  If you're knocked down, you can charge, you're hitting someone, you can charge, you're in the air, you can charge, the round is starting, you can charge.  Make use of this.

You can also charge two directions at once.  All charge attacks start with either back or down, so if you press downback, you're charging both at the same time.