Basically I have a mesh that I would like to change into a terrain so I can put textures on it ect.
I am afraid this will not be possible at your current knowledge level.
However, you can search asset store for object painter or something, in the editor/shaders category.
A more lengthy answer would be - learn about objects Topology, UVs, and then look into texture splatting technique
Terrains are (usually) planes that have been molded into shapes. Depending on how much you know, you could take the mesh in Blender, unwrap it and paint it in that - if you would like a static terrain. Then, you export back to .fbx and place both the texture and new mesh into your project, apply a Mesh collider to it as long as the vertex count is low enough.
If you want to do this dynamically, that in itself will take more work. It may be a better option to use the native terrain system in Unity, as that allows you to paint onto it directly in the editor.