Where are you trying to get it? Surface controller or Razor?

In razor you can do 

@{
Member current = Umbraco.Members.GetCurrent();
}

In a controller you can get the context in your constructur and use that

public class MyController : Controller
{
    private readonly IUmbracoApplicationContext _context;
    public MyController(IUmbracoApplicationContext context)
    {
        _context = context;
    }

    [HttpGet]
    public ActionResult DoStuff()
    {
       var member = _context.Security.Members.GetCurrent();
    }
}