I always advise my clients to dress simple, comfortable, and neutral. You’re investing in images to keep forever so don’t follow any fashion trends and go for something classic. Avoid shirts with large logos or writing, and go with soft materials. Comfort is really important because you’ll be moving, walking, and need to look relaxed. It can also be nice to bring an extra sweater or shirt if you’re not sure, or want a quick clothing change!
Look at images you like online and think about your background. You don’t want to contrast too much, for instance black on snow becomes really flat and stark. Blending in should also be avoided, for instance green jackets with forest, or sandy colored pants on the beach. Talk to the other members of your family and plan your outfits together, they do not need to match, but should be similarly themed.