PetaPoco is very easy to use and very fast. I will give you a simple example in order to see how to use it. I have not tested the code below but I am sure that it is going to work for you. Inform me if you have any problem.

First you have to define your model.

using Umbraco.Core.Persistence;
using Umbraco.Core.Persistence.DatabaseAnnotations;

namespace DataExample
{
    [TableName("Customers")]
    [PrimaryKey("CustomerID", autoIncrement = true)]
    [ExplicitColumns]
    public class Customer
    {
        [Column("CustomerID")]
        [PrimaryKeyColumn(AutoIncrement = true)]
        public int CustomerID { get; set; }

        [Column("Name")]
        [Length(50)]
        public string Name { get; set; }
    }
}

After that you have to make sure that the table will be created automatically and this can be done with the following code.

using Umbraco.Core;
using Umbraco.Core.Persistence;

namespace DataExample
{
    public class RegisterEvents : ApplicationEventHandler
    {
        protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
        {
            var ctx = applicationContext.DatabaseContext;
            var db = new DatabaseSchemaHelper(ctx.Database, applicationContext.ProfilingLogger.Logger, ctx.SqlSyntax);

            if (!db.TableExist("Customers"))
            {
                db.CreateTable<Customer>(false);
            }
        }
    }
}

Finally you can use the following class in order to access your data.

using System.Collections.Generic;
using Umbraco.Core.Persistence;

namespace DataExample
{
    public static class CustomersRepository
    {
        public static IList<Customer> GetAll()
        {
            UmbracoDatabase db = Umbraco.Core.ApplicationContext.Current.DatabaseContext.Database;
            return db.Fetch<Customer>("SELECT * FROM Customers");
        }

        public static Customer GetByCustomerID(int CustomerID)
        {
            UmbracoDatabase db = Umbraco.Core.ApplicationContext.Current.DatabaseContext.Database;
            var Data = db.Fetch<Customer>("SELECT * FROM Customers WHERE CustomerID = @0", CustomerID);
            if (Data.Count > 0)
                return Data[0];
            else
                return null;
        }

        public static void Insert(Customer customer)
        {
            UmbracoDatabase db = Umbraco.Core.ApplicationContext.Current.DatabaseContext.Database;
            db.Insert(customer);
        }

        public static void Update(Customer customer)
        {
            UmbracoDatabase db = Umbraco.Core.ApplicationContext.Current.DatabaseContext.Database;
            db.Update(customer);
        }

        public static void DeleteByID(int CustomerID)
        {
            UmbracoDatabase db = Umbraco.Core.ApplicationContext.Current.DatabaseContext.Database;
            db.Execute("DELETE FROM Customers WHERE CustomerID = @0", CustomerID);
        }
    }
}