I saw in one story okulumda, but when I want to say I at home, I say Ben evdeyim. What is write first: suffix for location, or possesive suffix? Please help me!

Hi there. The correct version really depends on the context.

okulumda => At my school

okuldayım => I am at shool.

Same with your example:

evdeyim => I am at home.

evimde => At my house

Remember 

noun + case-ending + personal ending

(okuldayım)

is not the same as..

noun + personal possessive ending + case ending

(okulumda)