Your kitten's first collar should be a safety collar made for cats and that will pull away easily in case the collar gets snagged. It should fit with enough slack to get two fingers in between the collar and her neck, but not loosely enough to slide over her head. Be sure to check the size every few days while your kitten is growing.
You can help your kitten get used to her collar by initially only putting it on her for short periods. If she seems uncomfortable or struggles and scratches at it, don't be concerned; it should only take a few days for your kitten to get used to having it on. When she begins to ignore it, you can leave her collar on all the time.
Remember that your kitten can easily get lost, especially in her early weeks, so it's important that you attach an identification tag to her collar. This tag should include her name and your contact information.
You can further help to identify your kitten if she becomes lost or is stolen by getting her a microchip implant. A microchip implant is an effective and easy way of linking your kitten to you. It involves a simple insertion of a tiny integrated circuit the size of a grain of rice under your pet's skin where it can't be seen, but can be read by an RFID scanner. The scanners allow animal shelters and animal control centers to quickly identify lost pets and reunite them with their owners. Please refer to the section on microchip implants for further information.