Hi,
Clip-in extensions can be a great, if temporary, way of adding volume and length to your hair.
You're absolutely correct to go for human hair extensions as you can treat it as your own hair, use heat and style them as you would your own hair.
If your hair is very fine you would be advised to use small - medium clips (just so they dont show through your hair). Many people prefer to make there own clip-in extensions so they can use the correct size clips and match the hair correctly.
Clip-in extensions need to be applied everyday (if you want them in that often). They do not need to be washed as often as your own hair - they only need "freshening" up as and when depending on how ofter you use them. Always wash and condition using a mild shampoo and conditioner and always give them a good brush before washing.
They are a good introduction to other types of extensions as they will give you a good idea on how to maintain human hair extensions if you go for other methods of attachment in future.
Hope you enjoy them!