So the thing is both the exam are literally poles apart and if you have not decided properly about your career prospects take time as it will need some introspection.
Based on just exam basis the broad difference can be summarized below.
Now CAT tests you on just 3 aspects namely verbal, logical and quantitative aptitude but CLAT has more sections and a good weightage for legal portion and the level of difficulty for quant is very low and also a very insignificant weightage is there so aspirants usually ignore the difficult aptitude portions. This will not happen in CAT as all sections carrying equal weightage requires one to focus on all the sub topics and have more depth knowledge when compared with CLAT.
Preparation wise first be really clear as to where you really want to go and then pick one domain and start with simple books or study material from any good institute and follow it religiously