If you are not comfortable in the lotus position, try an unpadded straight backed chair. Without arms is better. Keep you feet flat on the floor, with your arms at your side and your hands on your lap The importance is in aligning the chakras so that energy moves freely.
I can’t really tell you how, you just have to kind of feel it.
Full lotus is the most effective meditation posture.
However, it really takes sometime to get to there.
You didn’t mention how long you have been practicing meditation.
It took me 3 years of practice from half lotus to full lotus.
I can easily feel that the energy (Chi) goes much smoothly in the full lotus posture.
Here is a link of some basic meditation instructions.
Some stretching exercises (Yoga) will certainly help to loosen your legs, etc.
Just be patience.