|Oh yes! The difference is just letter "h"!! But, when should we use 'a'? And 'ha'?|
First, 'a' is a preposition. It is the equivalent to English "to" or "for" words.
For example: "Ana va a la playa" ("Ana is going to the beach")
But 'ha' is a verb. It is the third person, singular - verbo Haber. This form, 'ha', needs a second word, what's called a past participle, in order to form a compound tense, the Present Perfect tense (Indicative). When verb Haber works this way, we say it is working as an Auxiliary verb.
But let's see an example: "Juan ha comido demasiado". ("John has eaten too much")
So, in this last sentence, the verb is "ha comido", where 'ha' is the auziliary verb and 'comido' is the past participle of "comer".
So, in few words:
'a' is a preposition and 'ha' is a verb.
Hope this helps!