Hi Mi Yeon, 'a talk' is natural, like you talked about something, but 'a conversation' seems organized, like a set topic from a teacher in school: 'today's topic is..' :) 'there's nothing I can do but..' or 'there's nothing for it but..' are ok, but 'I can't help (myself) but ..' is a different meaning, like it's your automatic action that you can't prevent! :)

Both "talk with" and "conversation" are very similar. "I can't help but..." means something like "I can't stop myself" "I can't help but wait" - implies, you are find yourself waiting, maybe you are trying to distract yourself but can't help counting down the minutes as you wait. "There is nothing I can do about it but wait." - means the the situation is out of your control. "When baking a scones, once you put them into the oven there is nothing you can do but wait 15 minutes for them to rise." - You can not do anything to make the scones cook quicker.