Ranua is only about 80km from Rovaniemi and you just have to follow the one and the same road to get there..not hard at all... it's almost impossible to get lost! You can go there by car or by bus. Buses go many times a day and you can check the best bus for your timetables. (website) I won't stop you if you really want to walk or go by bike but I don't recomment it! :)
Words you may need to know:
lähtöpaikka = departure place (Rovaniemi)
määräpaikka =destination place (Ranua)
lähtöpäivä = departure day
kaikki päivät = all days
hae = search
eläintarha = zoo
linja-auto asema = bus station
lentoasema = airport
If you don't have any special wishes about where in Rovaniemi you want to hop in, just press the button "HAE" again after you press it the first time and there comes this red text:
"Löytyi useita paikkoja. Jatka ehdotetuilla paikoilla painamalla 'Hae' tai valitse muu paikka pudotusvalikosta."
If you want to hop in for exaple in Ruokasenkatu or at the airport and hop out in the zoo or in Simo you may choose them from the lists you'll get.