Check the church's own photos first
A church website or recent social photos can usually tell you more than a generic dress-code article. If most people are in jeans and casual shirts, you probably do not need formal clothes. If services look more traditional, slightly dressier clothing may help you feel comfortable.
Casual is common
Many contemporary churches are very casual. Clean jeans, chinos, a simple dress, skirt, polo, sweater or plain shirt will usually fit without drawing attention. You do not need to buy a special church outfit to attend.
Traditional settings may be dressier
Some Catholic, Orthodox, Anglican, Presbyterian and other traditional congregations may have more people in dresses, collared shirts, jackets or other dressier clothing. Visitors are still generally welcome even if they arrive more casually.
Choose comfort and modesty over anxiety
You will have a better first visit if you are not constantly adjusting your clothes or worrying about your appearance. Wear something you can sit, stand and move comfortably in during a service.
If you are still unsure, choose the middle
A clean, simple outfit that is one step above your most casual clothing is a safe middle ground. The goal is to make it easier to walk through the door, not to pass a fashion test.