To make a clean breast of it
means to make a confession and
to get it off your chest
means to unburden yourself of what is making you sad or heavy-hearted.

Don’t tell the devil all your plans.
Sometimes it is better not to go into too much detail about what you are going to do.  Your plans may be thwarted.

Don’t wash your dirty linen in public.
If there are things you are ashamed of, there is no need to tell everyone.

To bandy something about
comes from ball games and is usually applied to spreading stories

Between you and me…
…I am going to tell you a secret.

Between you, me and the gatepost…
…I won’t say anything, neither will the gatepost, so it’s your fault if the secret gets out!

Don’t mention it.
This is often said by someone who has just been thanked.

Not a dicky-bird.
A dicky-bird is a small bird.
Although I grew up in Greater London, I don’t know much Cockney rhyming slang (where a word is replaced by something which rhymes with it).  This one is “Don’t say a word”. 

I’m in the dark
I don’t know what’s going on.

I didn’t know how to broach the subject.
Broach means to begin talking about.  One of the next two phrases might be a way to start. 

I’ve something to say to you.

I’d like a word with you.

A word in your ear…
is probably a quiet word.

To be all ears…
listening attentively.

Show some enthusiasm
is advice which might be tempered by the following expression.  Enthusiasm is an interesting word.  It comes from a Greek word meaning possessed by a god, or inspired.

In John’s Gospel Chapter 14 verses 15 to 17 Jesus makes a promise to his disciples about the Holy Spirit.

Don’t get carried away.
Misplaced enthusiasm might need to be discouraged.