When you want to say "I think" in Japanese, you add と おもいます at the end, after whatever it was you thought (spaces used here for clarity only; there are usually no spaces in Japanese writing). So to literally say "I think that is good" you would say それ は いい だ と おもいます
But the direct translation from English is almost never going to sound like natural Japanese, so you might want to look for common phrases and expressions to study.
I think if people were just talking, you'd probably hear something like いいですね！がんばってね - which translates roughly to "That's great! Go for it!"
Good luck with your studies. :)