how can i fix my renault laguna?

my renault laguna is 1996 model 2ltr rt today it was working fine but this night i think they are going to start problems i turned the engine off and when i came to start it again it didnt want to start i checked the battery leads they are ok the thing is the dash board lights they look bright vand when i try to start nothing but i start it by push and it ok then but now i am not shure if i change the battrey or the starter ,the imobiliser can you help me thanks