Every time they complain about things in life, you can answer with I suffered that myself but thanks to God He helped me through it. Typically, that opens the door about who this God is and so forth.
But be careful here at this point. Just say what God has done for you. And use that by your proof he exists.
EXAMPLE: I used to suffer from fear...but during a period of my life, I chose to accept God and believe in Him. Eventually, I asked God to help me get past this fear. Now today, I don't even think about it because I never fear. And I know I could have never made myself feel that way. I know it had to be God, because everything/everyone else failed and nothing helped until I gave it to God...
It's your sincere testimony that opens their emotions. And the Holy Spirit will be there to make your sincere testimony be alive, as the H.S works on them.