Hello. I have a few questions. When speaking German, what is the normal words to use in conversation/what is the difference between these:

ihr and du?

seid vs. bist vs. sind?

einen  vs. ein?

es tut mir leid vs. entschuldigung?

Also, what is they in German?

How do you know if a noun is masculine or feminine? 

When saying guten tag is the g pronounced like a k in english or a g in english?

When I look up how to pronounce German letters some examples are t is prounounced 'tay', q is pronounced 'koo' etc., does that mean when I say a German word do i pronounce all of  the actual word tay when I see a t or is it just pronounced as a t, q ect. in english? 

I hope my questions are easy to understand, thank you for whoever answers them! It'll help me learn a lot! :)