The diffuse material is the basic color of the material. The normal map creates the illusion of depth, is a teqnique used in primarily games
to makes meshes look like they are more detailed than they actually are. Specular controls what specular highligts look like, “the shiniest areas where the ligt hits the object”.
You need to plug in these in the right spot on the material node.
You dont put a texture directly on a object. You apply the material to the object. Then in the material you choose wich
textures it should use. Just plugging in the diffuse will give the object color ie the image appears on the object. Then you can choose if you want to use a nomal map for illusion of depth and specular for more realistic lighting of it. The object needs to be
UV mapped tho for the textures to be placed on it properly. UV coorinates are are point on a 2d plane that correspond with the objects verticees wich are point in 3d space. Ive never uv mapped a object in UE4 and dont know how, maybe someone else can tell you how. I do it in Maya. Do you have a 3D modeling program like Maya, Max or blender? Im not sure how much you know about 3D graphics based on your question, fell free to ask more questions if u need more help.