6 So then, just as you received Christ Jesus as Lord, continue to live your lives in him, 7 rooted and built up in him, strengthened in the faith as you were taught, and overflowing with thankfulness. — Colossians 2:6 7 NIV
The most important thing to be thankful for is salvation. So, from the very moment we are saved, our hearts should begin to be filled with gratitude. From that point on, all the blessings that we receive daily should continue to fill our hearts with thanksgiving and praise for our Savior and Lord, Jesus Christ. To the extent that we follow the leadership of the Holy Spirit, our evil thoughts and desires are changing to good thoughts and desires. There has/will come a day when we should be overflowing with joy and gratitude because of the goodness and greatness of the God that we serve. Each day should bring less complaining and more reason to give God praise and thanksgiving. The praise and gratitude that fills our hearts will be reflected in the words that we speak.
4 Enter into His gates with thanksgiving, and into His courts with praise.
Be thankful to Him, and bless His name.
5 For the Lord is good;His mercy is everlasting,
And His truth endures to all generations. — Psalm 100:4, 5 NKJV