If you want to get to Montmartre or the viewing platform at the Sacre Coer, you can take the metro. For tourists, a (multi-)day ticket is recommended anyway, so you are always mobile and save your feet. To get to the Sacre Coer, take the metro line 12 to the Abbesses stop, and once you have reached this station, take one of the two lifts (don’t walk the station up, it feels like 500 stairs going up in a circle). Be sure to wait for the next lift (!!) you walk straight into the first street on the left. And walk and walk and follow the street, even next to the smaller square with the nice street cafés, until you see the Sacre Coer at your top left.
Taking the public Funiculaire de Montmartre to Sacre Coer hill
Yes, the Sacre Coeur hill is quite high up. You can either climb the stairs directly in front of the Sacre Coer or you can climb the last few metres with one of the Funiculaire de Montmartre, a funicular or inclined lift. I would have thought that they would open their wallets for the tourists and charge extra for the short ride. But I was wrong. Just insert your regular metro ticket into the turnstile, and you can take the next train up in no time. Super nice you don’t have to pay extra, and so convenient!
To guide you directly to the ground station of the inclined lift, navigate yourself to: Funiculaire Gare Basse – find it on Google Maps here: https://goo.gl/maps/twCZ749RAtbdTVft7
How to check out the public transport timetable in Paris
To find out when the metros and funiculars in Paris operate, you can either do it on site. Or, more conveniently, online. Google Maps is a great resource in the meantime, otherwise I can recommend the official website iledefrance-mobilites.fr/en which is also available as smartphone apps for both iOS and Android. Search in your App Store/Play Store for “Île-de-France Mobilités app”.