First, think about why we pray to God. We can pray to God in order to praise Him or to thank Him. But most often, we probably pray to God to ask Him to help us in some way. Jesus taught us to do this when he taught the Apostles (and us!) The Lord's Prayer [Matthew 6:9-13].

When we pray, we often pray alone - in the morning when we wake up or before we go to bed at night. But when there is something particularly important we want to pray for, we will ask others to pray with us or for us. A couple celebrating their 50th wedding anniversary might invite their family and friends to a Mass to help give thanks to God for their happiness. Or if a family member is sick, you might ask friends to pray with you for that person's recovery. If we are happy, we want others to share our joy. If we are worried about something, we want the comfort and support of others.

Who do we ask to pray for us? Generally we ask those who are closest to us - our parents, children, brothers and sisters, or close friends. There are prayer circles where people share their prayer requests and pray for each other. All of these people that we may ask to pray for us or with us have something in common. They are people we believe are good examples of our faith; people whose prayers on our behalf will be effective.
The Church teaches us that the saints in heaven do hear us when we ask them to pray for us [
Catechism of the Catholic Church, 2683]. The saints are free from all sin and in the presence of God for eternity. They have no distractions or concerns of their own and can devote their entire selves to prayer. St. James tells us, "The fervent prayer of a righteous person is very powerful." [James 5:16]  If we are looking for others who can pray effectively for us, can we make a better choice than asking the saints?

To learn more about praying to the saints, you can read
The Intercession of the Saints or Praying to the Saints on the Catholic Answers web site.