As to Jesus being God the Son, the following is written in the New Testament of the Bible: "But about the Son (Jesus) he (God the Father) says, "Your throne, O God, will last for ever and ever, and righteousness will be the scepter of your kingdom." (Hebrews 1:8 NIV, words in parenthesis are mine)

We see in the above verse, God the Father calling His one and only son Jesus, God. In fact, the first chapter of the New Testament book of Hebrews speaks of the divinity of Christ Jesus. The same is true of the following verses in the Bible: John 1:1-3; 1 John 1:1-2; Colossians 1:15-17; Hebrews 1:3, 8-12; Philippians 2:5-6.

In answer to the second part of your question, “If there is only one God, then how can Jesus also be God?” we see the earliest reference to the plurality of God in (Genesis 1:26). This is what is written: "Then God said 'Let Us make man in Our image, in Our likeness...'"  It’s important to note, that God alone created mankind. Neither, angels, or any other created being, assisted.

God has revealed Himself as God the Father, God the Son (Jesus) and God the Holy Spirit.  As it has been said: There is only one true and eternal God, but in the unity of the Godhead, there are three coeternal, and coequal Persons. They are the same in spiritual substance (or essence) but distinct in subsistence (personhood). God the Father, God the Son, and God the Holy Spirit are not the same Person, however, the Three make up the one true and eternal God. (John 14:26)