Are there any differences between them? Do they both mean "I think that..." in English? Thanks in advance.
Yes, they're not interchangeable.
"je trouve ça" is followed by an adjective, and can be translated by "I find it ..." rather than by "I think that ...".
Je trouve ça beau = I think it's beautiful
Je trouve ça bizarre qu'il soit parti aussi vite = I find it weird that he left so quickly
"je trouve que" is followed by a clause. that is often omitted in English.
Je trouve que tu devrais faire plus d'efforts = I think you should make more efforts
Je trouve que ça te va bien = I think it looks good on you
Note that you can "transform" sentences with je trouve ça to use je trouve que by adding c'est before the adjective, making it a clause.
Je trouve ça bizarre
Je trouve que c'est bizarre
