You can build an expression tree which contains calling of the parameterless constructor of the specified type using Expression.New method. For example, you have the following code.
List<string> list = new List<string>();
Here is the code that is required to build the same functionality using expression tree.
NewExpression newExpression =
System.Linq.Expressions.Expression.New(typeof(List<string>));
Console.WriteLine(newExpression.ToString());