Triadhi - As Su.Ki. has explained, you should use the gerund in your examples. "Playing guitar is fun". "I love/like/dislike/hate playing guitar".

Paul - you mentioned a future event ("at first you did some stuff,but now you want to play guitar instead ...") so you should use the verb "want to", or its more polite form "would like to", e.g. "Now that I have finished my homework I would like to play my guitar" or "I have finished fixing the car and now I would like to wash my (very dirty) hands". I understand that it may be hard to hear the "would" in spoken English, as it is usually shortened to "I'd like to..." but it is important to use it as it has a very different meaning to "I like".  

Hermann - In your second question (addressed to Paul), you ask whether a sentence using like+gerund is more ambiguous than like+infinitive. No, it isn't. In most cases, verbs that express like/dislike - which include also e.g. "love", "hate", "prefer", "don't mind" - take the gerund. Here is an example of ambiguity: "I like football".  This sentence is grammatically correct, but the meaning is ambiguous. I have not said whether I like playing or watching football. "I like playing football", on the other hand, is not ambiguous. It is very clear. True, it does not suggest that I "know how to" play. I may play very badly, or I may play brilliantly, but whether I am good or bad at football, I do like playing it. 

As Su.Ki. mentioned, there are a few instances where an infinitive can be used - e.g. to express what you think of as a good habit but which is not necessarily enjoyable. But in general, I would suggest using the gerund.