well if its a general "how to breed plecos" question, its not very black and white im afraid. too many different species, with different spawning needs and trigger requirements etc. try to pick a couple of plecs to narrow it down, there are plenty of different ones to choose from, some are easy, some are hard, and some havent been sucessfully bred or fry raised yet (due to sizes, not any information on there breeding habits, incorrect information etc). to sex them, you can look at their genital papilla (considered most sure fire way) but is not definate in itself and there are usually a few traits to look at. Other ways can include the presence or extent of odontodal growth, general body structure of the pleco, actual visual patterning or presence of bristles on ancistrus etc, some of which shown here
http://www.plecofanatics.com/forum/index.php?page=sexing_zebra_pleco . Again, it depends on the species of pleco how to go about it and getting the easiest and most reliable way of sexing them. Also sexual maturity differs between species. im afraid you'd have to be a bit more specific, and find a particular species out of the many hundreds of plecos there are known to be.
If you are looking for a starter pleco for breeding, most ancistrus or bristlenose species would be your best bet. They are easy to sex, readily available (although not necessarily common with some species) and quite easy to start breeding. I've put a lot of information on MFK (use search or if cant find them can link them for you) in various places about breeding them, and there are lots of articles out there on many of the 55+species in this genus.