i have 3 question's

To get past the pysducks you have to get the speacial potion from cynthia and then use it on them. As for the gym leaders I don't remember I beat it ages ago. Hope this helps

You cannot challenge the Hearthome Gym Leader, Fantina, until you beat Maylene in Veilstone, beat Crasher Wake in Pastoria, and eliminate the Galactic Grunt in Celestic Town (the secret Psyduck potion is tied in here--find Cynthia at the Lake Valor entrance after defeating Wake). Only then can you challenge Fantina. After that, you should have gotten--and learned--Surf, so go to Canalave City, which lies west of Jubilife City, and beat Byron, the Canalave Gym Leader, to use Strength. I forgot which Pokemon the Galactic Grunt uses, but he should be weak if you have trained your Pokemon well. As far as the Gym Leaders, here are the Pokemon you will be facing and their levels w/types:

Maylene (Veilstone): Meditite (27: fighting/psychic), Machoke (27: fighting), and Lucario (30: fighting/steel)

Crasher Wake (Pastoria): Gyarados (27: water/flying), Quagsire (27: water/ground), and Floatzel (30: water)

Fantina (Hearthome): Drifblim (32: ghost/flying), Gengar (34: ghost/poison), and Mismagius (36: ghost)

Byron (Canalave): Bronzor (36: steel/psychic), Steelix (36: steel/ground), and Bastiodon (39: rock/steel)

These are the third through sixth Gym Badges in this order and you gain the ability to use Fly, Defog, Surf, and Strength, also in this order. Train hard and good luck!