How to make my picture into A4 size?

You should use something like GIMP photoshop 2.6 or Adobe illustrator. open a A4 blank template, then open your pic, copy paste onto the A4 template and scale the pic in the A4 template to size. Dimensions for an A4 size in pixels is 2480px x 3508px....be sure to change the dpi to atleast 300dpi for decent resolution.