Before engaging in Yoga, some people find it useful to focus, relax and calm their mind.  People use different forms, but many people find it useful to repeat a prayer or mantra of some sort.  Repeat these with deep feelings and a strong will. Within a short period of time, you will see the benefits. It is also a good idea to sit in a meditative pose, while doing this.

Although any prayer can be used, there are some traditional Sanskrit prayers that are used as chants. Try this one and the English translation is below as well:

Sanskrit

Om Om Om
Om Thryambakam Yajaamahe 
Sugandhim Pushti-Vardhanam 
Urvaarukamiva Bandhanaan 
Mruthyor Muksheeya Maamruthath. 

Om Namah Sivaaya Gurave 
Satchidaananda Moorthaye 
Nish Prapanjaaya Shaanthaaya 
Niraalambaaya Thejase. 

English

We worship the three-eyed One who is fragrant and who nourishes all beings. 
May he liberate us from death for the sake of Immortality, 
Even as the cucumber is severed from its bondage of the creeper. 

Om! Salutation to the Guru 
who is Siva (auspiciousness), 
who is the embodiment of Existence/ Knowledge/Bliss, 
who is free from world consciousness, 
who is peaceful, without support and 
Self-effulgent.