1.-Should I make an icon to represent the button?
Icons can be really confusing, ambiguous, and they add to the time it takes the user to interpret the design. The general recommendation is that if you can say it with words - you should. You should definitely not base the interface on icons only, but you may consider adding them next to the text.
Like anything in UX - it depends and there are exceptions. Icons can add to the aesthetics of the design, albeit while adding visual noise. Also, if there aren't many icons, experienced users may get accustomed to them and may locate a function by the icon rather than text. But the interpretation time for a single word is typically similar to that of an icon - so no real gain here if the label is one word.
2.-Is it OK to display "attending" instead of "I´m attending"?
Yes it is. It is implicit that you are the one to attend and no one else. That's why you don't see buttons labelled "I'm OK with this" but just plain "OK".
3.-Should I add some decoration to the uploaded image? (Gradients, etc...)
Refrain from adding any graphics unless it serves a clear purpose. Gradients, in general, add visual noise while not helping with any semantics. Even their aesthetic is now heavily disputed in the age of flat design.
4.-Is the uploaded image user-friendly? (easy to understand...)
I don't understand what is the uploaded image and how is it differ from the icon. Also, is your plan to put the number of people attending on the same button saying "attending"/"interested"? Because that may be confusing.