1 Peter 2:1-3 Therefore, rid yourselves of all malice and all deceit, hypocrisy, envy, and slander of every kind. 2 Like newborn babies, crave pure spiritual milk, so that by it you may grow up in your salvation, 3 now that you have tasted that the Lord is good. (NIV)
Milk is an essential part of a baby's diet. Without milk they would die.
When we are "born again" into the family of God we become babes in the faith. Without spiritual milk we would die. This milk is the word of God.
Whether you are new to the Christian faith or have been a Christian for a number of years, the progress of your spiritual growth depends, to a degree, on how much time you spend with the word of God - The Bible.
Physical and spiritual growth have many similarities. A Baby (and even adults) normally require their food immediately they wake up (or as soon as possible). The same should also be true for spiritual milk. As a new Christian (and even a more mature one), you will discover that your hearts desire is to know more about God. God has revealed Himself in his word. Take time to read the word daily. Preferably let this be the first thing in the morning or the last thing at night.
Never starve yourself of the word. If you do you risk loosing your appetite altogether. This could result in stunted growth or death (i.e. backsliding from the faith). A Christians who refuses to live constantly on the word of God will remain a spiritual baby (1 Corinthians 3: 1-4). The qualities mentioned in 1 Peter 2:1 (quoted above) are symptoms of a spiritual baby.
Reading the word daily is how you drink spiritual milk. The gospels (Matthew, Mark, Luke and John) are a good place to start reading. However if you are to grow strong you will need more than the milk of the word (Hebrew 5: 12-14). You will need solid food. Getting solid food out of the word involves more than just reading it.