美味しいお店をみつけましたって英語でなんて言うの？

I found a fantastic restaurant!
Found - to discover (We discovered that we both are from Osaka.)
Fantastic - really good (This is really good music.)

I found a great place to eat!
Great place - Somewhere that is really good.
To eat - to enjoy a meal.

You could also say,
"I found a great place to get bubble tea." (Get - to buy/purchase)
"I found a great place to buy an iced coffee." (Iced coffee - cold coffee with ice cubes)

"I found a good restaurant"
This is implying that you have 'found' a good restaurant, 'found' is the term used when discovering or finding something new to you. 'found' to be in the past tense context.
"I came across a good restaurant"
To 'come across' something implies that you have discovered or found something, this is in a more general and casual sense/saying.

There are a few ways to say "I found..." including:
I discovered - to find something unexpectedly.
I stumbled upon - like falling over a rock you didn't see, finding something unexpectedly.
I came across - appearing.

There are also a few ways to say "A good restaurant"including:
A great restaurant / an awesome restaurant / an exquisite restaurant - better than just good.
A nice restaurant / a cool restaurant - another way of saying good.
A cute restaurant / a pretty restaurant / an instragrammable restaurant- nice in appearance.

When saying that you like something then you would say it is 'good' or ;great' so you would simply say 'i found a good restaurant'
you could also say 'i found a great place to eat' as a restaurant is where you go and eat a meal

When writing in your diary that you found a good restaurant, you can use one of the above comments. You may like to use more emotive adjectives than 'good'.

I found this excellent restaurant - This would mean you found an extremely good restaurant. Example : " I found this excellent restaurant, You should try it".