Translation:The first to arrive will find the best places.
English doesn't tend to double the subject the way Italian or French can. In speech, we might say 'The first to arrive, THEY will find the best places' for emphasis, for example to disagree with someone who says the people coming last will, but it's quite informal. Normally if there's a subject we don't double it with a subject pronoun.
You're trying to translate word for word and it doesn't work that way. In English the verb is ''to arrive'', but in Italian, you can consider the verb to be the equivalent of ''arrive'' alone without the ''to''. So you need to add ''ad'' for it to make sense, otherwise it's like saying ''the first arrive'' in English.