TFS API – How to fetch work item(s) from specific Team Project

You could try something like this for getting all WIs within teamProject:

WorkItemCollection workItemCollection = workItemStore.Query(
     " SELECT [System.Id], [System.WorkItemType],"+    
     " [System.State], [System.AssignedTo], [System.Title] "+ 
     " FROM WorkItems " +
     " WHERE [System.TeamProject] = '" + teamProject.Name +
    "' ORDER BY [System.WorkItemType], [System.Id]");

And this to get a specific WorkItem ID:

WorkItem workItem = workItemStore.GetWorkItem(555);