Hi i am thinking of rent a server but i want to make some changes on it but i dont know how to add plugins or create plugins, can i get help here?

You can try posting in one of the game forums to see if anyone in the community has experience with it.

If you're looking at GT Premium, keep in mind they are self-managed so you would be expected to know how to configure it or be able to figure out how to do so. If you think you'll have trouble running it, you may want to look towards a provider that will assist with customization like GameServers.com