Help translating sentence

лучше бы ты делом занялся.

I kind of know what each of these words mean individually but I am discovering that knowing the dictionary definitions of words in a sentence is really only the tip of the iceberg when it comes to translating said sentence. The thing is more than merely the sum of its parts. делом is so broad a word that I'm not sure which of it's meanings is relevant here and I do not know why it is in the prepositional case. I feel as though the sentence means something like "You had better do the thing" or "You'd better do that" Any one who can shine some light on this would be appreciated. Thanks.

делом is in instrumental, as заниматься demands instrumental. Compare: заниматься плаванием, заниматься благоустройством, ...
Заняться делом means to stop being idle, start doing something useful.

'лучше' is 'better', 'бы' is 'would'. So, 'лучше бы' is common starting collocation with meaning 'It would be better (if)...". Tricky part for english speakers is absence of 'it'. It is common practice in russian to omit words where they can be restored from context. But also words can be removed at all in some cases. Another example is english collocation 'They say...' which is just one word 'Говорят...' ('say...' with 'plural' mark) in russian. 'They' is not just omitted, but removed. 'Они говорят...' ('they say' literally) in rusian has another meaning: 'These specific guys we are talking about say that...'. Removing of 'they' is the sign of 'unspecific nature of subject' and plural suffix of verb means 'there are many of them who are unspecific/unknown'. Meaning of 'говорят...' is shifted to 'There are some sayings which claims...'. As you can note, in 'It would be better' this 'it' is also unspecific thing. There is no specific object or subject representing this 'it'. So we do not say it at all. Structure of language with a lot of suffixes and endings allow us to understand relation of words and their roles without some words. I suppose it reqires some reshape of mind to fully understand such things in another language (it applies to english too).

лучше бы ты занялся - you'd better (do some work); я лучше займусь - I'rather (do some work)