Hi H_safieh. Welcome to the forum.
It's best to use fresh gardenia seeds to get the best germination rates. Soak them in water for 24 hours before planting. You will want a loose mix like peat moss and vermiculite or you can use perlite or sand in place of the vermiculite. Plant around 1/4 inch deep and keep them at about 75 degrees for best results. It can take a while for gardenia seeds to germinate...anywhere from 4 weeks to 3 months. Best of luck and post some pictures when your seeds start to sprout.
https://MoonShadowsFarm.com Good Eats & Treats from the Pocono Mountains
"Go confidently in the direction of your dreams. Live the life you have imagined." ~Henry David Thoreau