On my experience. For a first time it is comfortable to use CONCEPT (SCHNEIDER ELEC.company)or Isagraf (CJInternational company )PLC soft.
It is not difficult to use. Both products are created by IEC1131 standart and consist from 5 languiges (ladder, functional block diagrams,sequential function chart,structure text, instruction list). Also its have simulators (you can use it without PLC) and consist from many math instructions (CONCEPT has fuzzy logic blocks , for example). You can get a training with many math on PC. But I beleave you can get free demo version of that only.
Evgeny