You should get this: The Waves Renaissance Compressor plugin.

It does a nice job, and is real easy to use.

Basically what you want is the vocals to not be super loud, then super
quiet. The compressor will smooth that out for you.

So what you do is put the compressor on the "insert" of the vocal
track. Then lower the "threshold" on the compressor controls about a
centimeter. Then watch the gain reduction meter. It should light up a
little very often. And whenever there is a big blast of vocal sound,
it should light up a lot. The more it lights up, the more it is
clamping down on the volume to make sure it doesn't get too loud.

If your basic signal is very quiet, you will want to boost the input
control on the compressor. Keep messing with the input level and the
ratio control until you see the lights go on and off like described
above.

There are other controls like "release" and "vintage" and "ARC". Just
set them to vintage or ARC and then leave them alone and concentrate on
the input level and ratio controls. That's a good way to go for you
since you are new at this.

There are many compressors and limiters, but the Waves Renaissance
Compressor is affordable, easy to use, and it's hard to get a bad sound
out of.

Contrast that with the Waves L1 limiter. It's aggressive and will
clamp the sound down, but it starts making the sound of the track hard
and annoying very soon, and very often.

The Renaissance Compressor will give you the smoothness you need
without the harsh sound added to it.