Clothing
No special clothing is required to ride a bike. Comfortable clothes that you can move around are fine.
Anything you wear that could get caught in the chain or wheels of a bike needs to be tucked in or tied away.
You should invest in a set of inexpensive waterproof/breathable over trousers and top. There are also rain capes that are popular with many people. In reality, you very rarely need rain gear as it tends to rain for only a few minutes and you can just stop for a coffee and cake until it passes.
Helmet
If you choose to wear a helmet it is essential that it fits correctly and that the straps and buckles are adjusted.
This means it must cover your forehead and not move about. The chin strap should snugly accommodate two fingers between the buckle and your chin.
A badly fitted helmet will offer almost no benefit and may introduce its own risks. Helmets should conform to British Standards BS EN 1078:1997