Try stillness. Think it's hard? Start with just one minute. Observe how you feel. Next, try two minutes. Again, observe how you feel. Next, increase the time to five minutes. After becoming comfortable with five minutes, double the time. Eventually, you may be able to go for 20 minutes or longer.
Remember, meditation doesn't have to be something hard to do. And it certainly shouldn't feel like a chore. It's not a race to see how fast you can increase your time. Slowly ease into it. Do what works for you!
If you do choose to sit, make sure you do appropriate stretches beforehand. Tao Yin yoga poses are extremely beneficial for increasing flexibility in the muscles and tendons.
Use props, if necessary. There's no shame in using a little assistance. Increasing the height of the hips just by an inch or two can lower the knees and make all the difference. Props can include a folded blanket to sit on or place under your knees, a cork or foam yoga block, or a Zafu meditation cushion. Also, try experimenting with various sitting meditation postures.
Make sure that your spine is comfortably straight. You shouldn't be hunched over. You also shouldn't be straining your back, causing pain. The seven-point posture of Vairochana is a good guide.
Once you're in the zone, continue to practice. You'll be glad you did!