Very Happy to Come  Here

I saw (see) a video last night, and in it the man promoted (in the video introduced) this website, so I have joined. (come here.) I want to improve my English and I'd like to help (the) people who want to learn Chinese :D .Anyway,come on! let's fight (fighting) together!

In the title, we normally use initial capitals for the important words.In your first sentence, you are referring to a past event (last night) so you should say 'I saw'. I have suggested using 'promoted' which means advertised; 'introduced' is normally used when you meet someone for the first time - a mutual friend may 'introduce' you to one another. You can also say 'I have joined italki'. Finally, I am not sure 'fighting' or even 'fight' is the correct word. 'Fighting together' has the meaning of 'struggling / hitting each other' in this context.   

Perhaps better to say 'let's help one another' / let's learn together'