This is not a server based question, but I'll be happy to answer it and try to guide you.
First, type "minecraft.net" into your address bar (also referred to as search bar).
Once you are directed to the official MineCraft website, you can now continue getting your skin changed!
Look in the TOP RIGHT corner of the website, you will see 2 options that say "Log In" and "Help".
Click "Log In" and you will be directed to a page that looks similar to this:
Enter your email associated (if you aren't verified for Mojang, please do so before starting this process.) with the account. Enter the password, and press "LOG IN".
After signing in, you will see your email in the TOP RIGHT corner, click on it. 2 options will be listed- "Profile" and "Log Out". Click "Profile".
You will then be directed to a page that is titled "CHANGING THE WAY YOU LOOK IN-GAME ON PC/MAC"
You can now change your skin by first selecting "Steve" or "Alex" as the base. (Steve has 4 pixel arms, Alex has 3 pixel arms.)
Then upload your desired skin and you're done! It will usually take a few seconds to update.
If you have any more issues or problems, message me at any time.