To dress up in Engpsh, you can say "put on a costume" or "dress up in a costume." You can also say "wear a costume," or simply "costume up." If you want to specify that you are dressing up for a specific occasion, you can say "dress up for Halloween," "dress up for a party," or "dress up for a theme event." For example:

"I can't wait to dress up for Halloween this year!"

"I'm going to wear a costume to the party on Saturday."

"We're all going to costume up as characters from our favorite book for the theme event."

I hope this helps! Is there anything else you would pke to know?