One of my coworkers always wants my help, at first I was happy to help her but now I am tired of helping her. She always thanks me for my help but she never mentions my help to our boss. Because of that I don't want to help her anymore but I cannot say no to her and I cannot give the reason. What is the best way to speak with her?
It sounds like this is becoming a real time drag for you - so is definitely something you'd want to address. However, you need to be careful how you do this. Excessively blunt answers will make you look like the villain in the piece while, on the other hand, excessively soft/cautious responses might re-direct her on individual occasions but are unlikely to see a change in behaviour over the long term.
First of all you want to make sure your manager knows what's going on. Bring it up indirectly. Rather than "Karen constantly needs help and I have better things to do" (which is a direct complaint about your co-worker) it would be smarter to approach them about the consequences of the time spent helping her. Something like "Just FYI, X might be a little late as I've had to spend a bit of time helping Karen with Y". If your manager would expect her to be able to do Y without any hand-holding they can begin to ask questions as to why she requires your assistance.
Secondly you want to try to address her behaviour. I don't know your exact roles - perhaps you are a Senior Thingey and she is a Junior Thingey and you're expected to mentor and coach her to some extent. In this scenario a certain amount of questions are unavoidable.
What you are trying to avoid are unnecessary requests for assistance. These are largely
Repeat questions: Basically this is a question you have answered before but she hasn't remembered the answer. In this instance, suggest she starts a wiki or document outlining how to handle these items. Then, she can refer to that the next time she needs the answer.
A lack of confidence in her abilities: This is where she might be 80% certain she knows what to do but doesn't have enough confidence and self belief to carry on with it. In this scenario, rather than seeking an answer, she is seeking confirmation that she's on the right path. Gradually building out a wiki or process document she could refer to would be helpful here too. In addition, rather than answer the question directly, spend a little time guiding her to the answer with questions of your own. What do you think you need to do? What's the first step? By letting her answer it herself she will learn that she knows more than she maybe feels she does and help grow her confidence.