blue bubble imessage

hello,

i am an active iphone user and my friend recently got an iphone. so we wanted to message each other using the imessage app provided by apple.

we tried to message each other but its costs us money and i heard we can message over wifi with no money loss. please tell me how to switch to the blue messaging (using wifi)

here is how the text appears as of now. i have done everything i could to use imessage instead of text message.

pls reply the earliest.

thank you.