We need to know how your saddle & seatpost are fastened on your bike. For full security you need to secure both. Have a look at the images and click the relevant buttons to move on to the next step.
Saddle - Hex Bolts
Most Saddles on modern bikes are fastened with hex bolts. Sometimes 1 bolt, sometimes 2. A hex bolt is fastened with a 6-sided (hexagonal) tool - hex wrench/Allen key.