The first week is about reading the room — dress slightly formal until you know the actual norm.
A blazer you can remove if needed
Starting slightly more formal than you expect to need is safer than the reverse. A blazer is easy to take off once you've gauged the actual office dress code.
A versatile shirt that works dressed up or down
Choose something that looks fine whether or not you keep the blazer on, since you'll likely be adjusting your formality level once you see how others dress.
Trousers that aren't jeans, at least at first
Even at casual companies, erring slightly more formal in week one reads as respectful. You can relax into the team's actual norm once you've observed it.
Comfortable shoes for a day full of walking and introductions
First weeks involve a lot of office tours, introductions, and standing around. Comfort matters as much as polish.
Subtle, professional accessories
Keep accessories understated until you've got a feel for the office culture. It's easier to add personality later than to walk back an overly bold first impression.