Before you go further, choose a theme that has a “Theme Editor” (variously titled such as “Theme Options”, “Edit Styles”). Choose a theme that allows you to change the header image AND the font colours(StripedPlus isn’t only one, is it?).
Step One: Title your blog.(do not use your name)
Step Two: Write your own one sentence blog description.
Step Three: Get an image from www.freefoto.com.
Step Four: Use Photoshop to put all layers together. Export the image, keep it less than 20k. Small .png is best.
Step Five: upload your image, find its url and copy/paste.
Pay attention to the imagery in the photo, synch with your blog title/description. Pay attention(eye dropper) to the colours in the image and edit your font colours. Choose a “unique” font in Photoshop for your blog title/description. Scale the final result to fit the fixed height/width of the header in your theme.