How should i back up my data on the phone which is not compatible with the latest version of whatsapp

I was using an old phone which wasnt compatible with the newest version of Whatsapp(updated on 4th February),so i wasn't able to use the app for 2-3 months and did not back up my chat history / account history for a long time.So will i reveice all the messages and the chats on my new phone which i didnt recevie in that period of time?And how should I back up my previous data?