It’s the first thing people see when they land on any page of your website. Just as you make a first impression when people meet you in person, your header is also the first impression people have of your blog. Ideally, you want people to look at it and immediately know what your site is about. It should build a connection with your reader by ‘telling a story,’ and that story will depend on the type of blog you have.
If it's a business blog, your story will probably be more on the serious side so your header should reflect that. But if it's a personal blog and your personality is fun and creative, then let your header show that.
Regardless of the type of blog you have, there are several things to consider when redesigning your header:
Size - shoot for a header image that isn't more than 250 pixels high. The header is important but you don't want it so big that it's taking up valuable above-the-fold real estate. In fact, many modern blogs and websites are using smaller headers than that, some as small as 150 pixels or less. If a small header will work for your blog, then go for it, because a small header leaves more room for content.
Image - if you're going to use any sort of picture in your header, it's really important that the quality of the image is the best possible. One of the quickest ways to ruin a great looking header is by using poor quality images in it.
Text - decide what text you're going to include in your header. Make sure your site name is in the header. If you have a tagline that goes with your site, then put that in the header too. Just don’t get carried away with fonts. You want them to be readable, and use no more than two to avoid confusion.
Visually appealing - we're all visual creatures so we're drawn to things that are attractive to us. Make sure your colors go together, that your fonts are clear, that the text is readable, and that the images convey the message you are trying to send. Remember, your header will set the tone for the rest of your site.
Keep in mind that you can always outsource this part of the process. If you don't have enough time or experience to mess around with creating a header, then it would make more sense to let someone who knows what they're doing handle it so you can focus on other aspects of the makeover.