Maths obviously, Perhaps further maths or statistics or something similar if you college offers it. Economics is available for some as A level, but not sure how helpful it is.
Check UCAS and see if Economics courses have any requirements, and some universities will specify this.
If there is a specific area of economics that you think you'd like to go into as well, it might be worth giving a try, so for example if you want to do political economics, politics or history might be an option. Or perhaps in business, then business studies.