Look at the code snippet found at this link for find command and execute here:
http://help.arcgis.com/en/sdk/10.0/arcobjects_net/componenthelp/index.html#/Find_Command_and_Execute_Snippet/0049000000n0000000/
Code snippet:
public void FindCommandAndExecute(ESRI.ArcGIS.Framework.IApplication application, System.String commandName)
{
ESRI.ArcGIS.Framework.ICommandBars commandBars = application.Document.CommandBars;
ESRI.ArcGIS.esriSystem.UID uid = new ESRI.ArcGIS.esriSystem.UIDClass();
uid.Value = commandName; // Example: "esriFramework.HelpContentsCommand" or "{D74B2F25-AC90-11D2-87F8-0000F8751720}"
ESRI.ArcGIS.Framework.ICommandItem commandItem = commandBars.Find(uid, false, false);
if (commandItem != null)
commandItem.Execute();
}
Replace the commandName from the following link. (example “select by rectangle”)
http://help.arcgis.com/en/sdk/10.0/arcobjects_net/conceptualhelp/index.html#/ArcMap_commands/00010000029s000000/
Call this method as:
FindCommandAndExecute(mApp,"esriArcMapUI.SelectFeaturesTool");