How do i know what the major and minor products of the reaction between Hydrogen Bromide and Propene would be?

We would use something called Markovniov's rule. Essentially, the H+ ion (or proton) that is attracted to the electron dense double bond of the alkene, is added to the carbon that has the most Hydrogen atoms bonded to it. This creates the more stable carbocation intermediate (in this case a secondary carbocation) which has a lower activation energy, and therefore a faster reaction, hence a more favoured product. In this case, 2-Bromopropane is the major, and 1-Bromopropane is the minor product