Articles always seem tricky in French, but with practice they are simple! In the first example the indefinite article 'des' is used - this means that the sentence is talking about new shoes in general. In contrast the second sentence uses 'les', the definite article, to describe the new shoes, showing that it is referring to specific new shoes. Imagine you are having a conversation about a pair of shoes you have seen in a shop window with your friend, and you want to say that you would love to buy those particular shoes.
Note to remember: even though the shoes are plural, the first example is 'de' instead of 'des' because this is the rule when an adjective comes before the noun. If the person just wanted some shoes, they would say 'J'aimerais DES chaussures', but because the adjective 'nouvelles' precedes the noun here, you must use 'de'.