1.
when i learn hiragana, i found つ its call tsu (tt/ss/kk/pp)
what mean of the parenthesis?

--> We have two types of つ, the standard one and the small one, it's っ. It is used (tt/ss/kk/pp) and with other letters of the words from overseas.

Ex) きっぷ=kippu(ticket) ちょっと=chotto(a little) アップロード=appuroーdo(upload) キッチン=kicchinn(kitchen) 

Also it used a lot when we conjugate the verbs and adjectives. 

Ex.) かう(buy) かった(past form/informal) かって(て-form)  

       うつくしい(beautiful) うつくしかった(past form)

2.
for word ko こ
i found it little hard to see difference when side with しor い
cause sometime i mistake read it as ni に
how to avoid that
--> In my opinion, the only way is practicing. When we started to memorize the alphabets, the same thing happened to us, for example, "q, p, g", "b, d", "t, f", etc but I think there is no way except practicing.