What is difference between aç and açık?

What is difference between şuan and şuanda,  what this mean?

How to say childhood friend?

Aç has different meanings both as a noun and a verb.

For example,

Kapıyı aç -> Open the door

Ben çok açım -> I´m very hungry

While Açık is an adjective,

Evinin kapısı açık  -> Your house´s door is open.

Çoçukluk arkadaşı is the translation of childhood friend.

As for the şu an thing, I think a native might be better than I am.

Hope this helps!

Şu an and Şu anda nearly same things and you can use one instead of another almost always. 

Şu an meşgulüm.

Şu anda meşgulüm. 

 both same thngs and doesnt sound weird to a native