It's not easy to want to help people, because you don't always have the skills or time to do so. And people can unintentionally ask for a lot more than you should give.
Whenever I come across this situation, I try to ask myself 3 basic questions.
I don't have the education to help with those issues. So I shouldn't try to help; I'd probably make it worse.
Of course I can give an answer: "Find a professional". But, it's usually not the answer they seek, even if it's the one they need.
Even when I am qualified and the problem is relatively small, I don't always have time or energy to help. It's not easy to say "no" when you know you could help. You also deserve time for yourself!
Fortunately, since you know how it works, you can often link them to a website that helps them out.
Would you want to do your job for free? Is it fair if someone asked you to spend hours without anything in return?
Consider that cost is not only money, but time, effort and involvement.
Sure, sometimes helping is very nice to do by itself. But, more often than not, there is a cost for you involved. Is the person who is asking someone who has done many things for you? Is there a balance?
I hope these help you to say "no" when you should, so that you can say "yes" when you want to.
You can't help everyone, nor are you qualified to do so. Save your energy for those that deserve it (including yourself!) and help them instead.