Hyvä ystäväni tarjoaa minulle aina ruokaa. // I think the verb "tarjota" (to offer) is a bit better here than "antaa" (to give). There is a slight difference in nyances, similar to what you can see in the use of those English equivalents. We give food to those that really need it to show our generosity but we offer it to those that we want to pamper or show friendliness or sociability. right?

Se on yleensä makeisia, jota hän antaa minulle. // Yes, you can say "karkkia" instead of "makeisia". The former is more inofficial than the latter. And then here I am acting against my own advice in the previous correction. I suggest using the verb "antaa" (to give). As I have seen, with candies the verb "antaa" is used more often without strong connotations, especially in young people's language. Translating "to share" into Finnish sometimes is a bit tircky. In pure finnish you don't use the expression "jakaa jonkun kanssa" the same way that you use the english verb. Instead you use "antaa", "tarjota", "kertoa" and other verbs.

En näe häntä kuin vasta ensi viikolla, mutta huomenna menen ostamaan hänen suosikkikarkkiaan. // here häntä is not tail but partitive case of the pronoun "hän" (he/she). Also "karkkiaan" is partitive case + there is the possessive suffix in the end "..aan".

Annan ne hänelle maanataina.

Hän pitää Kit-Katista. // minä pidän, sinä pidät, hän pitää ...

=) Ostan niitä paljon. (I will buy a lot of them)