I find this game difficult to find power berries unlike Back to Nature. So can someone please tell me how to get all of them

You max out at 10 powerberries in this game. It is possible to get more then 1 power berry from a location. For example, if you won the horse race in the Spring, you get a power berry. If you win again in the Fall and still have less then 10 powerberries, you will probably get another Power Berry.

Where to find Power Berries:

quote westpole72

PBerry 1 » In your field
Simple enough! Take your hoe and dig around your crop field. The berry will eventually turn up.

PBerry 2 » Harvest Goddess
Take items you've grown on your farm to the Harvest Goddess. Toss the item into the water to make her appear. After throwing in an item for 10 days she'll reward you with a berry.

PBerry 3 » Turn in medals
When you bet on the horses during the derby race, you can win medals. Exchange 900 medals to Mayor Thomas to get the berry.

PBerry 4 » Win the Horse Race
Your horse will become an adult at the beginning of your second year. You can now participate in the Spring and Fall horse race! Yeehaaw! If the two of you come in first place you'll win the berry.

PBerry 5 » Frisbee disc winner
In your second year you can have your dog join in the Dog Frisbee event on Summer 01. The prize is a Power Berry, so practice throwing the frisbee with your dog when it becomes an adult in the Fall of your first year. You will also have to buy the Frisbee from Won for 5,000 G.

PBerry 6 » Fishing off the pier
When you upgrade your fishing pole to level 5 (mithril), use it at Mineral Beach in the Winter. You might just catch a berry.

PBerry 7 » Dig down into the mine
Next to the Harvest Goddess' pond is the mineral mine. Usually you go here to get ore for tool upgrades, but if you did down far enough you'll find a berry. Actually the berry is down on the 100th floor and you'll have to dig it up with your hoe once you get down that far.

PBerry 8 » Television Shopping
Once you've purchased most of the items off of the Saturday t.v. shopping show, they might just have a berry for sale. It's not cheap! You'll have to fork over 10,000 G.

PBerry 9 » Behind the Winter Mine
When it's Winter time the lake on Mother's Hill will freeze over. Walk across it and you'll see a small island in the middle of the lake. The island has the entrance to the Winter Mine on it, but if you walk behind the island and press the A button you'll find a hidden berry.

PBerry 10 » Dig in the Winter Mine
Pretty much the same as the berry that's hidden in the mineral mine, but you don't have to work as hard for it. The berry is only on the 19th floor this time.

quote DarkMist

each power berry you get (they are red and you can get up to ten of them) will add +5 stamina points permanently. That adds up to 50 more stamina than normal after you get 10. The blue berry adds +50 fatigue permanently, so its a good thing to have. Your character starts the game with something like 75 stamina points and 50 fatigue. Once you get all the berries this adds to 125 stamina and 100 fatigue. I'm pretty sure 1 swing from any tool except the cursed ones at regular power uses up 1 stamina or 1 fatigue point every time you use it, but when you charge them up they use more.