How to expose API endpoints in C# WebServices?

Question

// Model
using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using System.Web;

namespace CoinJarAPI.Models
{
    [DataContract]
    public class CoinJarModel
    {
        [DataMember(Name = "volume")]
        public decimal Volume { get; set; }

        [DataMember(Name = "amount")]
        public decimal Amount { get; set; }

        [DataMember(Name = "getTotalAmount")]
        public decimal GetTotalAmount { get; set; }
    }
}

// Controller
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Web.Http;

using CoinJarAPI.Models;

namespace CoinJarAPI.Controllers
{
    public class CoinJarController : ApiController
    {
        // GET: api/CoinJar
        public IEnumerable<CoinJarModel> Get()
        {
            var coinJarList = new List<CoinJarModel>();
            for (int i = 0; i < 10; i++)
            {
                var coinjarModel = new CoinJarModel
                {
                    // volume, Amount, GetTotalAmount.
                };
            }
            return coinJarList;
        }

        // GET: api/CoinJar/5
        public string Get(int id)
        {
            return "value";
        }

    }
}
// Interface
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace CoinJarAPI.Interface
{
    interface ICoinJar
    {
        void AddCoin(ICoin coin);
        decimal GetTotalAmount();
        void Reset();
    }

    public interface ICoin
    {
        decimal Amount { get; set; }
        decimal Volume { get; set; }
    }
}

Hi Team

 

I want to expose 3 endpoints that will do this following  and this is what i have currently, question where do i implement these endpoints? Controller or Interface, do i have create a Model? Need some advice

 

Add a coin

Get the total amount of coins

Reset the coins