God speaks to God?

Scriptures demonstrating the Trinity are so numerous in scripture that it is actually very good advice not to look for them. Rather, it is far better to learn a few you know well, and keep those in your back pocket, so to speak.

An excellent example I would recommend is Hebrews 1:7-8. This verse is triply lethal to anti-Trinitarian heresies, and it's actually fairly easy to find if you can't memorise the exact location. Just think to yourself "it's near the beginning of Hebrews", and it actually becomes more powerful evidence for the Trinity if you include Hebrews 1:1-6.

The first reason Hebrews 1:7-8 is lethal to anti-Trinitarian heresies specifically relates to Jehovah's Witnesses. These believe Jesus is a mere angel. Michael the archangel, specifically. But the verse explicitly distinguishes between the Son (i.e. Jesus) and the angels.

The second reason is because the Son is specifically referred to as God. This really doesn't leave room for interpretation. But more amazing than this is the third reason: Who is referring to the Son as God? God! God calls the Son God. Not only does this firmly establish the Trinity, but it also puts a lot of anti-Trinitarian misquotes into perspective. Yes, Jesus prayed to the Father. Yes, Jesus said "The Father is greater than I". Yes, Jesus said "no one is good but God" (though given that Jesus is good, this in itself is sufficient to prove, rather than disprove, His divinity). But God calls the Son God. If God calls the Son God, then it is entirely legitimate for persons of the Trinity to refer to themselves separately.

Thus, if I was to recommend just one passage to keep in mind when discussing the Trinity, it would be Hebrews 1:1-8. If time is short, just cut it to verses 7 and 8. This is far from the only Trinitarian Bible verse, but it is certainly a very effective one, as it leaves no room for interpretation.