How do you use "sweet" or "sweetheart"?

Can I say "sweetheart" or "sweet" when I meet a kind person. For example, when my friend gives me some sweets what can I say? Can I say, "Oh, thank you! you're so sweet"?. Or, when I talk about my friend to someone else, Can I say, "She is such a sweetheart"?
The words, sweet and sweetheart sound like words that lovers use.

Sweetheart:
a) You could call your girlfriend or boyfriend with this expression
b) or a little kid which gives you something to play with etc.
c) or you pet ;-)

but you should avoid using such word for people you just met / unknown people / bosses / (coworkers) 

"Oh, thank you! You're so sweet":
This is of course possible. You can say this to your friends.

"She is a so sweetheart":
The correct form would be more like this "She is such a sweetheart". So when you talk about this cute girl with your friends, you can use this expression. But it would be clear to your friends that you have developed a deeper relationship with her than normal. Or in other words you know here better.

Sweet and sweetheart sounds like a word that couples use?:
Sweet is a adjective which everybody uses. The latter should be clear to you from my further explanations.

Hope that helps!

How do you use "sweet" or "sweetheart"?

Can I say "sweetheart" or "sweet" when I meet a kind person? For example, when my friend gave me some sweets, can I say "Oh, thank you! you're so sweet"?. Or when I talk about my friend to another friend, Can I say" She is a such a sweetheart"?
Sweet and sweetheart sound like a word that couples (lovers?) use.

In the UK, one would generally use 'sweetheart' to talk about someone of the opposite sex. The way you have used it in your example is accurate. You could also say that someone was 'my sweetheart', which would mean that they are your lover or partner.

I would say that you could say that someone of either sex was 'sweet'. In your example, the word is accurately used.

Happy Christmas!

Rob