The first time I had to add code to my header I was terrified. I didn't understand what I was look at when I looked at the code on the header.php file and I was nervous to add something to the wrong place.
I've since learned a bit more about code, but I have also come across this awesome plugin - Insert Headers and Footers - that makes it so easy to do.
In addition to not wanting to add code to my header for fear of breaking my website I have also heard from numerous sources that adding code directly to your header.php file can cause issues when your site updates.
Using this plugin is so simple and easy to do and makes sure that you don't run into any problems down the line.
If you have any questions please pop them in the comments.