Breathing: Deep inhale, 1-2-3 seconds, hold it for a second then followed by deep exhale of about the same duration. But do not breathe through the open mouth, no need for that for now.
Posture: Sit up straight either on your ankles or cross legged. If that's hard, do not push it but use a rolled up blanket or pillow to make yourself more comfortable.
Then, gradually get into or changing from one to another perform some basic hatha moves, asanas.
Stay away from recently much promoted so called chakra yoga. Properly and gradually learned it can lead to a blissful state and great balance in life. But there must be a basic understanding as to what it is about and how and to what degree one wants to get involved in it.
Even for advanced Hatha Yoga there is sufficient information available to study and practice meditation and a few breathing techniques which includes a few so called mantra sounds which I've covered already.
There you are, something to start with. Pilate, a form of yoga was developed almost 100 years ago in the States, it uses some equipment to make some specific exercises a bit easier and more suited for certain parts of the body to develop more than others, but basically it is yoga or better hatha yoga related.
