What is Mandarin's conversational equivalent of English's "How interesting!" or "That's cool"
As for things like 
”I started taking guitar lessons" or "I saw that movie yesterday" you can say 嗯，感觉怎么样？（Oh， how do you like it?) to show your interest in the conversion.
Then they may say,不错（well）、一般（average）、糟（bad）,and now you are taking control of the conversion. You can comment on their response with few words and talk on something else.  

对...object...感兴趣=be interested in ...
If the object is somebody then it's a different matter:you have the tendency to love him/her.
有趣 seems not so native， people often say 有意思 instead. This show that they feel something is interesting. Besides, they say 搞笑 instead of 滑稽 or 有趣 when something is funny. They also say 太扯了 to say something is so ridiculous or amusing.
Some short expressions in English are widely accepted, such as good,what,cool,okay.  

cool can be translated into many Chinese，but it rarely has connection with 有趣. More often it's 帅 or 牛。  

You can use 有趣 to describe an activity as well a person.But here is one point：if you use it on somebody,you have to make it clear that you are saying he or she is humours rather than ridiculous.

Do not repeat one sentence over and over again, that just implies your mind has gone miles away from the topic.