There are many different programs you can practice your skills on, there are a few on-line plc editors this site has one but I know after this site upgrade the link did not work so unsure if it is still broken, however, perhaps a better way is to go to automation direct they have the click plc software but I don't think it has a simulator (perhaps it may now), there is the Do more (PLC) & CMore (HMI) that do have simulation abilities & I think to testing purposes it may also be free or perhaps time limited, it would be better than an on-line system if I remember correctly the Do more can simulate both the PLC & HMI so they interact with each other. there are some examples apparently bundled with the downloads. There may be others google is your best friend.
here is a link to it.