If God had predetermined everything, how can we be free? 

The world defines freewill as the ability to choose good or evil. This is not the Biblical view. In the bible, we learn that humankind is fallen and doesn't have the ability to do what is good and pleasing to God. Human freewill is fallen with a default gravitation away from God's will. 

Ephesians 1:3-8

Blessed be the God and Father of our Lord Jesus Christ, who has blessed us with every spiritual blessing in the heavenly places in Christ, just as He chose us in Him before the foundation of the world, that we would be holy and blameless before Him. In love He predestined us to adoption as sons through Jesus Christ to Himself, according to the kind intention of His will,  to the praise of the glory of His grace, which He freely bestowed on us in the Beloved. In Him we have redemption through His blood, the forgiveness of our trespasses, according to the riches of His grace which He lavished on us.

One day a group of theologians sat around discussing the doctrines of predestination and free will. When the discussion turned into a heated debate, they split into two groups. One man, unable to make up his mind which group to join, slipped into the predestination group. Challenged as to why he was there, he replied, “I came of my own free will.” The group retorted, “Free will! You don’t belong here! join the free will group” So he retreated to the freewill group. When they asked him why he was there. He responded, “I was sent here.” “Get out,” they stormed. “You can’t join us unless you come out of your own free will.” 

Video - The Predestination Debate - White vs Brown

Video - Chosen By God - R.C. Sproul

Video - Predestined for Hell? Absolutely Not! [#2065] - Adrian Rogers

With freewill, we all fail the test. In Adam all have sinned. 

Gen 2:16-17

Then the Lord God took the man and put him into the garden of Eden to cultivate it and keep it. The Lord God commanded the man, saying, “From any tree of the garden you may eat freely; but from the tree of the knowledge of good and evil you shall not eat, for in the day that you eat from it you will surely die.

Romans 5:12

Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men because all sinned