"Jak se máte?" means "How are you?" and we use this sentence after greetings "Hi!", "Hello!", "Good morning!" etc. So, we use this expression after the everyday greeting people, which we know already since while. "Teší mě" means "I am glad to meet you". We use this sentence, when we've just got acquaintance with someone new. For example: - Jak se jmenujete? (What is your name?) - Jmenuji se Janek. (My name is Yanek.) - Teší mě. (I am glad to meet you.)

I always used "Teší mě". These are very different statements, so the use should be obvious.