Conseiller mon amie puour visiter mla ville de Trento

Corrected a small typo and added an article before the city name. Also you cannot really use a possessive article here since you do not really own the city.
If you want to emphasize about the fact that this city is your hometown, then you could write something like "Conseiller mon amie pour visiter ma ville natale / ma ville d'origine, Trento".
Also, if you are adressing this to your friend, then it would be best use a noun rather than a verb in the title, and write "Conseils pour visiter la ville de Trento".

Chèrer Laura, ¶
je vais vous dire les meilleures choses à faire aà Trento pour la famille, parce que je sais que le prochain mois vous voulez venir ici pour une visite.

Laura is a female name so you should use the feminine form. Same thing goes for the word "choses"; for which you will modify the word "meilleures" accordingly.
Added the pronoun "vous" in the first part since you used the same pronoun at the end of the sentence. Please note that it is very polite, and that if you are adressing a friend, you could use the pronoun "tu", which would then become "te" ("Je vais TE dire les meilleures choses.... TU VEUX venir ici...").
Finally, when naming things to do at a location, you should write "à" and not "a" as it is used as a preposition and not a verb.

Trento est une tréès petite ville, nous pouvons marcher tranquillement dans la ville avecsans aucun problème.

Wrong accent on the word "très". When using a negative, you should use the word "sans" instead of "avec".

Le centre ville est ancien, comme toutes les villes d'Italie, i. Il y a le Castello del Buonconsiglio, qu'i est vraiment beau et avec un grand jardin.

Slightly changed the punctuation here, and removed the abbreviated form "qu'est" that is used orally but should actually be written "qui est".

AParmi les autres choses que vous devriez visiter esont le musée de la science et le parc aà còôté, (les enfants aiments les activités du musée; d). Dans la meême zone, il y a la bibliotheèque universitaire, opeu l'opéra de l'architecte Renzo Piano.

Since you gave multiple suggestions, added the word "Parmi" and an article at the start of the sentence. Also changed the conjugation of the verb "être" since we are using plural form.
Changed the conjugation of the verb "devoir" to the conditionnal form, since you are giving an advice. Using the word "devrez" sounds more like an order in this case.
Corrected a couple wrong accents (côté, même, bibliothèque, opéra) and slightly changed the punctuation.

Trento est la ville principale de la région Trentino Alto-Adige, laquelle est aà l'extreme nord-est du pays, en fait il y a nume : il y a en fait de nombreuses montagnes, et elles sont tréès voisinproches les unes des autres.

Corrected the accent on "à" for the same reason as earlier.
Changed the punctuation to match the context and moved "en fait" at another place in the sentence to sound more natural.
Added "les unes des autres" for better understanding since the initial sentence would translate to "the mountains are close"; so it is best to add something like "to each other".

Donc, si tu veuxvous voulez voir la ville tu as besoin d'une matin,ée; et je tevous conseille d'utiliser une journeée pour faire une excursion.

When giving a period of time, it is best to use the word "matinée" instead of "matin".
Corrected a typo in the word "journée".
I replaced the pronoun "tu" for "vous" since we used it at the beginning.