Lotus Position
Lotus Position is good but not required. For it, you need to use a pillow (zafu), as shown
here.
What's so special about Lotus? If sitting on a chair, for example,
you put one foot on the opposite thigh, it'll be an ambitious
position; if you do the same with the other foot, it'll be
super-ambitious so Lotus is a super-ambitious position.
Seiza
You can also sit in the seiza posture. If you find it's difficult, you can use a pillow, as shown
here.
Sitting on a Chair
You can just sit on a chair, as shown
here.
When You Do Zazen, Two Things Are Very Important...
(1) Your knees (or feet, if you're sitting on a chair) must be in firm
contact with the floor; why it's important will become clear later when
I'll describe how to do
the walking zazen (my zazen method that I practice.)
(2) As they say
here,
'The position of your torso is also very important, especially the
position of the fifth lumbar vertebra; keeping it in the correct
position will enable the spine to be curved correctly. The belly should
be without tension, and you should remain stable. My master used to say
that the curving should be such that it is like the ass is looking at
the sun.'
In other words, your tandem should be filled; here're some photos as illustrations:
http://the-1.ru/…/v-yalte-vyberut-korolevu-belly-dance_1.jp…
http://www.japan-sumo.ru/AKI2007/1kayo.jpg
http://abzac.org/wp-content/uploads/2013/01/201-192x300.jpg
I don't know who you look like, as for me, I'm somewhere in the middle :-D
Then you need to focus on your breathing. There are different opinions on how to breathe during zazen, I recommend breathing by
the bamboo method. Even better if you control at
the same time
the Point Seika no itten (you can pinch and hold it with the fingers) in the lower abdomen.
If you have any difficulties when doing sitting zazen, start with
the walking zazen, and only then move to sitting meditation.