Vocal Distortion on Recording

Am new to this forum...and am not sure if I'm posting in the
right place, or if there has been a previous post of the same question. Tried to do a search for FAQ's but unable to come up
with anything. or if someone could redirect me to the right
place to post, I would appreciate it.
.
Question: I am using a HD recorder / Tascam 788 and was trying
to lay down some Vocals, I'm using a standard SM58 Shure...all
I have for now.

But I get very bad distortion.

I set my slider to 'unity gain', bring up my 'trim' level of the input, and as I'm raising it, I hear distortion coming in around half open on the trim setting. with not enough boost in the signal to have
any real volume at all.

So I'm getting distortion on the very low end of the scale, even
before it's barely audible?

Anyone else experience this, not necessarily on the Tascam 788, but on any recorder or system...and how have you solved this
problem?

Any help appreciated.

I also thank all for allowing me to make this post possible.

Thanks

Vern