How to get data from json api with c# using httpwebrequest?

First you need a custom class to use for deserialization:

public class Item
{
    public string id { get; set; }
    public string name { get; set; }
    public string symbol { get; set; }
    public string rank { get; set; }
    public string price_usd { get; set; }
    [JsonProperty(PropertyName = "24h_volume_usd")]   //since in c# variable names cannot begin with a number, you will need to use an alternate name to deserialize
    public string volume_usd_24h { get; set; }
    public string market_cap_usd { get; set; }
    public string available_supply { get; set; }
    public string total_supply { get; set; }
    public string percent_change_1h { get; set; }
    public string percent_change_24h { get; set; }
    public string percent_change_7d { get; set; }
    public string last_updated { get; set; }
}

Next, you can use Newtonsoft Json, a free JSON serialization and deserialization framework in the following way to get your items (include the following using statements):

using System.Net;
using System.IO;
using Newtonsoft.Json;

private static void start_get()
{
    HttpWebRequest WebReq = (HttpWebRequest)WebRequest.Create(string.Format("https://api.coinmarketcap.com/v1/ticker/"));

    WebReq.Method = "GET";

    HttpWebResponse WebResp = (HttpWebResponse)WebReq.GetResponse();

    Console.WriteLine(WebResp.StatusCode);
    Console.WriteLine(WebResp.Server);

    string jsonString;
    using (Stream stream = WebResp.GetResponseStream())   //modified from your code since the using statement disposes the stream automatically when done
    {
       StreamReader reader = new StreamReader(stream, System.Text.Encoding.UTF8);
       jsonString = reader.ReadToEnd();
    }

    List<Item> items = JsonConvert.DeserializeObject<List<Item>>(jsonString);

    Console.WriteLine(items.Count);     //returns 921, the number of items on that page
}

Finally, the list of elements is stored in items.

A simplified version of Keyur PATEL's work.

static void GetCoinValues()
{
    string json = new WebClient().DownloadString("https://api.coinmarketcap.com/v1/ticker/");

    List<Item> items = JsonConvert.DeserializeObject<List<Item>>(json);

    foreach (var item in items)
    {
        Console.WriteLine("ID: " + item.id.ToUpper());
        Console.WriteLine("Name: " + item.name.ToUpper());
        Console.WriteLine("Symbol: " + item.symbol.ToUpper());
        Console.WriteLine("Rank: " + item.rank.ToUpper());
        Console.WriteLine("Price (USD): " + item.price_usd.ToUpper());
        Console.WriteLine("\n");
    }
}