First and foremost, the worshiper should be physically and ritually clean. The body and clothes must be free of urine, feces, blood (other than your own, which is forgiven), et cetera. However, sweat, mud, dust and the like are not impurities and need not be removed. To be ritually clean, the worshiper must perform ablution called wudu’ before praying. If the worshiper has major ritual impurity, such as sexual emission, he or she must perform a ritual bathing of the whole body, called ghusl. See Conditions of Prayer: Physical Purity for details. 

If after making ablution, one passes gas or does anything else to nullify the ablution, he or she must perform ablution again. If the ablution is nullified during the Prayer, he or she must leave the Prayer, repeat the ablution, and start the Prayer from the beginning.

Note that a woman who is having her menstrual period or postpartum bleeding cannot perform ritual Prayers until the bleeding has stopped and she has performed ghusl. But it is recommended that she stop what she is doing at the times of the Prayer and remember Allah and supplicate Him. She does not have to make up for the missed Prayers.

For more details you can read: Acts that Nullify Ablution and Things that Necessitate Ghusl

The Place of Prayer

The worshiper must have a clean place to perform the Prayer where there are no impurities on the ground. Also there should be no statues or pictures in front of him. It is not necessary to have a prayer rug or special cloth, but many find this convenient, especially in public places or at work.

What to Wear While Praying

A man must be covered from the navel to the knees when performing ritual Prayer. It is highly recommended that he also cover his chest and shoulders. A woman must be covered from head to foot with opaque clothing, with only her hands and face showing. It is best if she covers her feet while praying.

It is not necessary to have special clothes to pray in. However, most women find it convenient to have a one- or two-piece loose garment that they can put on over their other clothes when they pray at home.

The Adhan

The Adhan (the Call to Prayer) should precede the obligatory Prayer, followed by the Iqamah (a shortened version of the Adhan signifying that the worshipers have stood up and are ready for the Prayer). If the Adhan and the Iqamah are already given, (by others) and the worshiper is about to perform the Prayer in congregation, he can simply join the congregation. If he is going to perform one of the obligatory Prayers alone, it is recommended that he say the Adhan and the Iqamah before performing the Prayer.

For more details, read: The Adhan (The Call to Prayer). You can also Listen to the Adhan.