The absolutely critical thing to consider of motorcycle clothing is its safety. It is no good to be wearing jeans and a t-shirt, or thin jacket while on a motorbike. The harsh truth is that if, although hopefully not, you have an accident, these clothes will not protect you well at all.
Your head is the most important part of you to protect, in case of an accident, therefore a helmet is the most important piece of your equipment. Go to a local equipment shop and try one on, try ten on, try them all on until you find one that fits properly. It should not slip around when your head moves, even when it is unbuckled and should also be comfortable to wear for long periods of time. You can use a spoon to press down the foam a fraction if a helmet is otherwise a perfect fit, but it is important not to compromise in this area. Consider also visors – tinted visors can reduce visibility but will also help reduce glare from the sun. Consider carrying spares in a saddlebag so you can always be wearing the optimum type.
Once this is done, you can consider proper clothing. Looking good is obviously important but seeing as you’re on a motorbike, you’re going to be moving fast and hard to see clearly. Special motorbike leather jackets are tanned for strength and are much thicker than ‘normal’ leather jackets. Riding leathers should also be reasonably snug, because loose clothing can cause rashes or chafing. If the best quality leathers you can find aren’t your favourite colour, never mind – your safety is crucial!
Textile gear is also available but my personal preference is for leather due to the increased levels of protection that it offers.
Boots are quite simple to get fitted properly – after all, I am sure you have bought shoes before and the process is essentially the same. One thing to consider is the weather of the environment you will ride in. Do you need your boots to be warm or breathable or waterproof, etc.?