When should I use celownik?
The dative has the following main usages.
After verbs that govern the dative (as direct or indirect object). These include dać komuś coś (give someone(dative) something(acc)) ufać komuś (trust someone), wierzyć komuś (believe someone) and a few others.
With prepositions that govern the dative, the most common is przeciw(ko), there's also dzięki (thanks to) ku (toward - not that commonly used) and maybe a couple more
As the 'psychological subject' describing states that affect a person. In english you say 'I'm cold' in Polish that's 'zimno mi' (it's cold to me) because the cold is affecting you, there's also przykro mi (I'm sorry about that situation) smutno mi (it saddens me) and a few others
The book dominic pointed you to should also help