You don't need special clothes to start with, but you will need trousers rather than shorts, and often a long-sleeved top. Unless the weather is cold, you will want thin clothes so you don't get too hot. Trainers are fine for shoes, but don't wear anything too nice as they will probably suffer from undergrowth in the forest, as well as getting wet and muddy - this applies to clothes as well. Once you become a regular, you will probably want to buy some special orienteering kit, and some hard-wearing, grippy, o-shoes.
The only fairly expensive equipment you need is a compass, but you might be able to borrow one if you don't have one. GPS is not allowed. A whistle is advisable, or even compulsory, in steep or exposed areas to call for help if you fall over and injure yourself, and safety pins are useful for all sorts of things. If you come to larger events you'll soon get used to seeing the Ultrasport and Compass Point vans, both sell a large range of orienteering kit and equipment.
If the weather has been wet recently the ground may be wet, in which case it is advisable to take a change of clothes and shoes. You may want to take a picnic and a drink, although there is usually squash or water available at the finish.
There may not be any toilets on site, so please be prepared for this. There is also rarely any changing facilities, so don't be shy about changing in your car, everyone else will be!
Make sure you have enough cash with you to cover any parking and entry fees.