Catholics believe that there are three divine persons (Father, Son, and Holy Spirit) in the one God. Though the Father is God and the Son is God, yet the Father is not the Son. And so, the Son (Jesus Christ) can pray to the Father.
The following, from the so-called Athanasian Creed, should give you a better idea of the Catholic understanding of God as a Trinity:
Now this is the Catholic faith: We worship one God in the Trinity and the Trinity in unity, without either confusing the persons or dividing the substance; for the person of the Father is one, the Son’s is another, the Holy Spirit’s another; but the Godhead of Father, Son and Holy Spirit is one, their glory equal, their majesty equally eternal. Such as the Father is, such is the Son, such also the Holy Spirit; uncreated is the Father, uncreated the Son, uncreated the Holy Spirit; infinite is the Father, infinite the Son, infinite the Holy Spirit; eternal is the Father, eternal the Son, eternal the Holy Spirit; yet, they are not three eternal beings but one eternal, just as they are not three uncreated beings or three infinite beings but one uncreated and one infinite. In the same way, almighty is the Father, almighty the Son, almighty the Holy Spirit; yet, they are not three almighty beings but one almighty. Thus, the Father is God, the Son is God, the Holy Spirit is God; yet, they are not three gods but one God. Thus, the Father is Lord, the Son is Lord, the Holy Spirit is Lord; yet, they are not three lords but one Lord. For, as the Christian truth compels us to acknowledge each person distinctly as God and Lord, so too the Catholic religion forbids us to speak of three gods or lords. The Father has neither been made by anyone, nor is He created or begotten; the Son is from the Father alone, not made nor created but begotten; the Holy Spirit is from the Father and the Son, not made nor created nor begotten, but proceeding. So there is one Father, not three Fathers; one Son, not three Sons; one Holy Spirit, not three Holy Spirits. And in this Trinity there is no before or after, no greater or lesser, but all three persons are equally eternal with each other and fully equal. Thus, in all things, as has already been stated above, both unity in the Trinity and Trinity in the unity must be worshipped. Let him therefore who wishes to be saved think this of the Trinity. For his eternal salvation it is necessary, however that he should also faithfully believe in the incarnation of our Lord Jesus Christ. Here then is the right faith: We believe and confess that our Lord Jesus Christ, the Son of God, is both and equally God and man. He is God from the substance of the Father, begotten before the ages, and He is man from the substance of a mother, born in time; perfect God and perfect man, composed of a rational soul and a human body; equal to the Father as to His divinity, less than the Father as to His humanity. Although He is God and man, He is nevertheless one Christ, not two; however, not one because the divinity has been changed into a human body, but because the humanity has been assumed into God; entirely one, not by a confusion of substance but by the unity of personhood. For, as a rational soul and a body are a single man, so God and man are one Christ. He suffered for our salvation, went down to the underworld, rose again from the dead on the third day, ascended to the heavens, is seated at the right hand of the Father, wherefrom He shall come to judge the living and the dead. At His coming all men are to rise again with their bodies and to render an account of their own deeds; those who have done good will go to eternal life, but those who have done evil to eternal fire. This is the Catholic faith.
