A good transformer to start with is the 1033.
http://www.tandem-associates.com/lionel/lionel_trains_1033_transformer.htm
It will easily handle one train.

If you are looking in the future to possibly run more than one train on multiple loops, then a KW would be recommended.
http://www.tandem-associates.com/lionel/lionel_trains_kw_transformer.htm

For more than 2 trains, or if you will get accessories, than the ZW is king.
http://www.tandem-associates.com/lionel/lionel_trains_zw_transformer.htm

All have a whistle controller, with the ZW having two.

Larry