にする

にする is a structure that combines the に particle (a location/target), with the verb する. It means to 'make/decide' something in a certain way. The nuance with にする is that the speaker has direct control over the outcome. This is where the strong feeling of 'choice' comes from. The noun that comes before にする is the thing that is being decided.

In this example, although the common translation is 'to decide', it is actually much closer to the slang expression 'to do (A)' in English. This means that the literal translation is actually 'I'll do a beer', or 'I'll make it a beer'.

Sometimes the particle と is used instead of に. Although this has the same meaning, it changes the nuance slightly. When と is used, it takes away the feeling of being a direct decision. とする tends to be used in more formal situations, but this is only because と is less 'definite', so results in the phrase sounding less pushy.

If you were going to compare these two to an English equivalent, it would be similar to the difference between the following sentences.

に - I'll buy a burger. (A burger is the direct target of 'buy')

と - I'll go with a burger. (You aren't 'going' anywhere, it just sounds softer)

This grammar point may be used for more abstract nouns, and does not specifically need to be 'items'.