How do I know if a word needs a preposition and if so how can I tell which one to use?

As with English, prepositions aren't needed in every sentence, however many nouns, verbs and adjectives do require one. Remember: they will affect the rest of a sentence grammatically. However, we can talk about this later...First of all, it's worth familiarising yourself with a list of the prepositions which appear most frequently such as:bis (until, up to, by)durch (through, across)entlang (along)für (for)gegen (against, towards)ohne (without)um (around, about, at) – when talking about timeOnce you have done this, you can start to listen out for them or keep an eye out when reading and will surely pick up on when and how they are being used.Be careful, as the prepositions don't always correspond to the obvious English equivalent. One German preposition can have multiple English equivalents. For example an can mean at, in, on, or to—depending on how it's used in a German sentence. If you were going to say: You should look after your grandma. You might think, OK, 'look after', after, that would be the preposition nach because it means 'after' in English. Although, it may be instinctive to translate these prepositions literally, it's best to give it some thought and not go straight for the obvious option. You would actually use the preposition um which really doesn't have much to do with the English 'after' at all.It would be: Du solltest dich um deine Oma kümmern. (sich kümmern + um)When learning a new word, whether that be a noun, adjective or verb, be sure to consider whether it would need an accompanying preposition. If so, it's useful to learn the word and the appropriate preposition together, as one. This way, you will feel that they belong together from the outset and will be unlikely to use the wrong one.It's good to test yourself by having a whole list of words and scrambled prepositions on the other side. Use the cursor to draw a line between the words and prepositions which belong together.Just like with vocabulary it comes down to repetition, try to learn which prepositions go with which words of by heart, until they become ingrained in your mind and you just know what sounds right and what doesn't.Using an incorrect preposition can completely change the meaning of a sentence.
For example:
Ich gehe auf die Toilette - I'm going to the toiletIch gehe in die Toilette- I'm going in the toilet (stepping in to the toilet itself)
So it's definitely worth knowing your aufs from your ins!