It is new to me to be using specific words due to my gender!
So I'm a bit confused.

When should I (a woman) add the "a" ending to words?

For instance:
I go into a hotel and say: Мне нужен одноместный номер. (???)
I meet a (male) friend: Рада тебя видеть.(???)
I need some time to think: мне надо подумать (???)

Why is this right or wrong?
Is this decided by the gender of the person/thing that I am talking about, or by my own gender?

Hope somebody can explain and help with a couple of easy examples!