Salvation is God's free gift to us, but we must accept it. We can never make up for our sin by self-improvement or good works. Only by trusting in Jesus Christ as God's offer of forgiveness and turning away from our sin can anyone be saved from sin's penalty. When we turn from our self-ruled life and turn to Jesus in faith we are saved. Eternal life begins the moment one receives Jesus Christ into his life by faith.
Because God gives us eternal life through Jesus Christ, the true believer is secure in that salvation for eternity. If you have been genuinely saved, you cannot "lose" it. Salvation is maintained by the grace and power of God, not by the self-effort of the Christian. It is the grace and keeping power of God that gives us this security.