i would like to know how i can convert date time variable into string,numeric variable into string and how we can concatenate together two string.
There is no such thing as a "date time variable". Dates and time are stored as a structure (separate month, day, year, etc...) or as a number (seconds since Jan 1, 1974 for example). How is your date stored?
You can print numbers directly:
Serial.print(8675309);
You can concatenate strings in many ways, depending on how the strings are stored. If you have then in String objects you can use the addition operator to concatenate them.
You need to be more specific about what exactly you are trying to do. Provide code examples if possible.
Which kind of string?
1.array o char (string) tipical of C language
or
2. a String object (Arduino library) ?
If 1,
- number to string, use itoa() o snprintf()
http://www.cplusplus.com/reference/cstdlib/itoa/?kw=itoa
http://www.cplusplus.com/reference/cstdio/snprintf/ - concatenate strings, strcat() or strncat()
http://www.cplusplus.com/reference/cstring/strcat/?kw=strcat
For date/time @john also ask you to be more specific.
If date and time parts are in different variables, you can use snprintf()
char buffer [100];
snprintf (buffer,100, "%02d/%02d/%04d", dt.day,dt.month,dt.year ); // example 12,3,1969 -> "12/03/1969"