Udyr is tricky. Bad players who lose to him complain that he is just a stat check, yet when most people play him they fail spectacularly.
Playing udyr requires strong map awareness. For builds, I recommend items with CDR, movespeed, and tankiness. Movespeed is probably the most important, because he has no dash or wall hopping abilities. You need to be able to juke skillshots as well.
You also shouldn't limit yourself to one build. You need to adapt to your team and the enemy team. Sometimes you need a splitpush build. Sometimes you need damage to function as kind of an assassin. Sometimes you just need to tank and peel for your hyper carry ADC.
I'd tell you to watch Trick2G but that usually ends up in disaster for inexperienced Udyr players.
I managed 65% winrate last season with about 100 games. Because of the state of the game (ADCs soooo strong), that meant playing almost exclusively as a peeling tank and doing very little damage. I would even get mocked during wins for doing least amount of damage in most of those games. We won because I body blocked skillshots and stunned any divers or assassins. Even the ADCs I was saving would say things like "we won no thanks to this Udyr". I didn't like playing that way, but that's what it took to win.