You need an FTP manager to upload / replace / delete content into your website.
You need (not a must) programs such as Photoshop (expensive), The gimp (free)... to draw layouts, logos...
You need coding skills, knowledge to code the layouts you created with photoshop, or the gimp or any other designing programs (mainly HTML and CSS)
You need to upload the layouts that you coded to your website.
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Now, lets assume you download a layout from zymic for free... well just download it to your site, open the pages with a text editor (your favorite one...) modify the content inside with your OWN content. Modify the logo (usually in a PSD format... so you will need Photoshop :/) upload everything from your FTP and boom... batabang... your done