the best way to lean to program is to write programs
a low cost (about £15) development board is Microchip's microstick
**broken link removed**
is has an onboard programmer and a header to plug into a breadboard for you own circuits
Microchips MPLAN and C30 are free for academic use and can be downloaded from their internet site
Go to the PLC vendor's web site and download their manuals. The instruction set is usually a good place to start.
If you don't have a particular brand of PLC in mind then try zelio logic smart relays. The programming software is (at least was) free and can do simulation. Also a zelio plc can be comparatively cheap to buy