Standing on hills takes more effort than staying seated. If it's a very long climb, it can help to stand for a while, and if it's a very short steep-ish climb sometimes I will stand to power up it. Otherwise, I stay in the saddle, shift into the smallest cog (and sometimes the granny gear), and just take a slow steady pace up the hill. Keep doing it and you'll get stronger, so you can ride faster, ride longer or both.