I can't give you a single rule, but I can give you some advice. It is impossible to translate prepositions between one language and another. If you try to find a one-to-one correspondence between 'para' or 'a' in Portuguese, you will get nowhere.
What you need to do is learn short phrases in English, for example 'Let's go to the beach', in the same way that native English-speaking children do. Learn the whole phrase as a complete unit, rather than trying to construct it word for word by translating each word from Portuguese. That way, you can be fairly sure that 'Let's go to the mall' or 'Let's go to the party' is also going to be correct. Does that make sense?
A few more tips which might help. Remember the word 'to' indicates movement, whereas 'for' doesn't indicate movement. So, if you want to translate 'Vamos para Londres', you'd say 'Let's go to London', because there is movement involved. Whenever there is movement, you need the preposition 'to'. Go to work. Go to school. Walk to the park. Swim to the island. Drive to the restaurant. Fly to Miami. Always 'to'. The only exceptions are 'home' and 'abroad', which are adverbs, and don't use any prepositions. So, 'Vamos para casa' is simply 'Let's go home'.
'For' indicates purpose, rather than direction. For example, 'This present is for you' or 'I gave this watch to my mother for her birthday' or 'He received an award for good schoolwork'.
The other to/for confusion between English and Portuguese is the infinitive of purpose, 'in order to', or in other words, saying 'why' you are doing something. Here the Portuguese 'para' translates as 'to'. For example, to translate 'Vou a Nova Yorque para comprar roupa', you'd say 'I'm going to NY to buy some clothes.'
I hope that helps.