function onOpen() {
var file = SpreadsheetApp.getActiveSpreadsheet();
file.setActiveSheet(file.getSheets()[0]);
var sheet = file.getSheetByName("sheetName");
file.setActiveCell(sheet.getRange(rowNumber, columnNumber));
};

Where "sheetName" is the name of the sheet you want to move within, rowNumber the number of the desired row, and columnNumber the number of the desired column.