When it comes to an interview, no employer is impressed when their job candidate turns up looking scruffy. To look your best, men should wear a professional suit and women should wear either a skirt or trousers. However you decide to dress though, try to keep it plain and avoid distracting or bright patterns. Most importantly, remember to polish your shoes–yes, the employer will notice!
The word "his," is replaced by "their," because there was no sex indicated in the first sentence, so it would be more appropriate to use.
The word "them," is replaced because you are talking about "to dress," in general. Them is usually for more than one item or people.