Tea: A strong Assam or Ceylon black tea is often used when making chai. Black tea works well because it is not overpowered by the spices and sweeteners. That does not mean other types cant be used, in Kashmir chai is sometimes made with gunpowder green tea, and for a caffeine free variation Rooibos can be used.
Spices: All masala chai has ginger, cinnamon, cardamom, and clove as basic ingredients. Other "warm" spices commonly used are star anise, black peppercorns, fennel, vanilla, chocolate, nutmeg, cocoa, liquorice, mint, and rose.
Milk: Whole milk is used for its richness, but non-dairy milk (soy, rice, hemp, almond) will also do.
Sweetener: Don’t limit yourself to white sugar, though it is often used. Experiment with the caramelized notes from brown sugar, the more complex slight acidity of honey, or the healthy sweetness of agave. You may be surprised to find yourself using a lot of sweetener, the spices are rich, and need a lot to bring the flavors out.