You can go two ways with this: you can either choose a signifcator that is representative of the querent, or let the deck choose one for you.
In the first case, when the querent is not you and you don't know them very well, make an assessment by asking questions. Ask the person what she enjoys doing or where she would prefer to take a holiday. What languages does she want to learn and does she like to read? It is perhaps best to do this before the day of the reading so you can put some thought into selecting a significator card.
In the second, ask the querent to shuffle the cards, and tell him or her to draw a card from the deck. Discuss the significator and relate the meaning of the card to the attitudes or personality of the querent.