The main difference between the Paris metro and the London tube is that the direction is shown by the name of the final station on the line, e.g. going northwards on Line 4, you look for 'Direction Porte de Clignancourt' rather than 'northbound'. As with the tube, you get a list of stations at the entrance to the platform, so you can double-check you have come to the correct one before boarding.
Changing lines is easy, just follow 'Correspondance' and look for the line/direction you want. ('Sortie' means 'exit')
As regards tickets, the main options are:
Navigo Decouverte, a weekly pass which runs from Monday to Sunday; you have to buy a one-off card for 5 euros (and you will need to bring a photo for it) which you then 'load' like loading an Oyster card
Tickets t+ sold singly or in a 'carnet' of ten, one ticket per person per journey(including changes)
Paris Visite: pass aimed at tourists, convenient but poor value for most people
Ticket Jeunes Weekend: one-day pass for young people under 26, valid Saturday, Sunday or Bank Holiday