If you are new to Tumblr's theme world, and have no idea where to find your customize page, we recommend that you follow the steps on this page: (Where's my customize page?).
On your customize page, you will be able to find the control panel on the left, and a list of options. If you see your 'Logo' option on your option list, click the camera button, and upload your logo. Notice that some themes may require you to adjust your image size before uploading it.
You may not have a logo yet, or prefer a text logo instead. Then, do it! You can always remove your logo image to use your text logo. For more advanced customizations, we recommend that you use Google Fonts. You may upload a logo that you created with your preferred Google Font, or use our Easy Google Font plugin.