Simple shader that outputs the color red.

HelloWorldUnityShader.shader

Shader "Hello World Shader"
{
    Properties
    {
        _Value("Value",Float) = 1.0 //Defines a custom float property called "Value"
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" }

        Pass
        {
            CGPROGRAM //STARTS THE HLSL CODE
            #pragma vertex vert //Telling compiler that the vertex shader is defined by the "vert()" function.
            #pragma fragment frag //Telling compiler that the fragment shader is defined by the "frag()" function.

            #include "UnityCG.cginc" //Imports unity-specific shader functions.

            struct appdata //Better name: "TEXTURE". Contains per-vertex mesh data.
            {
                float4 vertex : POSITION; // Vertex position.
                float4 normals : NORMAL;
                //float4 color: color; //RGBA colors. 
                //float4 tanget
                float4 uv0 : TEXCOORD0; //uv0 diffuse/normal map textures
            };

            //Everthing that is passed from the vertex shader to the fragment shader has to exist
            //inside of this struct.
            struct v2f //Better name "Interpolators"
            {
                //float2 uv : TEXCOORD0; //You can write anything to these channels, doesn't have to be "UV"
                float4 vertex : SV_POSITION; //Clip-space position
            };

            float _Value; //Need a variable to match the _Value property in the properties section. Automatically sets the value of the property.

            //Defines what data will be interpolated.
            v2f vert (appdata v)
            {
                v2f o;

                //Converting local space to clip space. 
                //If we didn't convert here, the shader image would be "stuck" to the camera.
                //This should be omiatted if you're doing a postprocessing effect like a vignette.
                o.vertex = UnityObjectToClipPos(v.vertex);
                
                return o;
            }

            fixed4 frag(v2f i) : SV_Target
            {
                return float4(1,0,0,1); //RED COLOR
            }
            ENDCG //ENDS THE HLSL CODE
        }
    }
}