My first day of italki

"I found the italki and I joined it today."

-> I found italki and joined it today. 

(Also it would be abit more natural to say: "Today I found italki and joined." )

I am learning English for a long time.

-> I have been learning English for a long time. 

When I watched so many people login here, I felt I was excited.

-> When I watched so many people login here, I felt excited.

(also it would be abit more natural to say saw or seen instead of watched because "watched" seems more suited to a literal/physical act, for example:

"I watched you leave"   

Also instead of saying "login" you could just say "on here"

-> When I saw so many people on here, I felt excited. 

"I meant they gave me a power."

by power I think you mean motivation, which would be more natural to say:

"it gave me the power"

or "it made me feel motivated" 

"Though I have a little time to learn English everyday,"

-> This is good and fine as it is but I think "Although" seems more suited to what you're trying to say here, also adding in "only" would emphasise this:

"Although I only have a little time to learn English everyday"

I try to keep learning English and the other language

-> more than one language = "language's" 

and I want to help someone who need help about Japanese.

• "need" -> "needs"

• "about" -> "with"