How can we access wp default functions from any plugin file

I came across this problem when I was creating a plugin. I cannot call(access ) wordpress default functions like wp_get_current_user(), $wpdb->get_results(), admin_url() etc. from my plugin files. I can access them from my main file of that plugin in which plugin description is given. But I cannot access them from other plugin files. At that time I heard that if we create a new file functions.php and put these code in that file inside a function I can access all wordpress functions. I did so. But now I cannot access that new function. How can I solve this problem? I think you guys understood what I said. Please help me.