Jesus was most likely crucified on a T-shaped cross with both vertical and horizontal beams like the ones familiar to the church today. However, the actual shape of the cross is never as important as the One who died on it – our Lord and Savior Jesus Christ.
Roman crucifixion could have been carried out in a variety of ways (T shaped, lowercase t shaped, X shaped, on a stake, or even upside down). However, in the case of Jesus, the details suggest a T shaped cross with enough room above His head to place a sign of Him as "King of the Jews" in three languages. Part of the importance of this discussion relates to the position of Jehovah's Witnesses who argue Jesus died on a pole or stake instead of a cross. The existing evidence suggests that some type of T shaped cross was most likely. However, even if it could be proven that a stake or pole was used at the crucifixion of Jesus Christ, the conclusion would not affirm beliefs regarding Jesus as a created being or denial of His deity. Ultimately, the exact shape matters less than who Jesus is and what His death accomplished—calling us to deny ourselves and faithfully follow Him (Matthew 16:24–25).
The reason there may not have been additional details regarding the exact shape of the cross in the Gospel accounts may be because the shape of the cross was not the focus—the One on the cross was. Jesus was nailed to the cross, indicating His hands being stretched out. Dr. William Lane Craig notes:"The description of Jesus' carrying his cross is consistent with the Roman practice of forcing victims to carry the crossbeam of the cross to the place of crucifixion. The nailing of Jesus' hands and feet to the wooden frame is suggestive. In John 21:18-19 the kind of death Peter would suffer is prefigured with the words 'you will stretch out your hands and another will gird you and carry you where you do not wish to go.' The second century pagan author Artemidorus similarly refers to criminals being 'crucified high and with hands outstretched' (Oneirocritica, I. 76. 35). The outstretched hands naturally suggest a lateral extension. Artemidorus confirms this when he later says, 'the cross is made of pieces of wood and nails like a boat, whose mast is similar to a cross' (II. 53. 3)." (from: http://www.reasonablefaith.org/was-jesus-crucified-on-a-cross).
It does not matter whether Jesus was killed on a T shaped cross or on a pole. What matters is that His death and subsequent resurrection conquered death and sin and revealed Jesus as the promised Messiah. Jesus is the sacrifice and substitution for our sins. Through His name alone we can believe and be saved, obtaining eternal life (John 3:16; 14:6; Acts 4:12; Ephesians 2:8-9). Jesus said, “If anyone would come after me, let him deny himself and take up his cross and follow me. For whoever would save his life will lose it, but whoever loses his life for my sake will find it.” (Matthew 16:24-25). The cross reminds us of Christ's death for us and reminds us of the call to faithfully follow Him, putting to death that which is of our sinful nature that we may be alive in Christ.