C# Read and write one line from textfile

[Snoze]

Hello, I'm trying to make an application with C# that writes and reads some lines of a text file, but I want to read one specific line and save the text in a string variable.
And the inverse process, save a string variable in a specific line of the text file.

Also i think that it could be done writing in the textfile "first= Here the text of the first line", "second= here the text of the second line"... and then, my application when want to read the first line, search the "first=" and read the text after =.

But I don't know do this :(

Thanks and sorry for my bad english.