The BBQ
Green = Correction
Red = Incorrect
Blue = Added words 

Yesterday was (is) Friday. We had (have) no classes in the afternoon, so we went to a farm for a BBQ. Yesterday was (is) a hot day and some of my classmates and I went to (were go to) buy a lot of (many) food (foods) as (and) something was needed to be cooked for on (in) the BBQ. We also invited our techer to join us. After eating the food, we ate a big watermelon and the watemelon is very sweet. The sun was shinning and we were happy.

(( Wah! I think you had a very fun day! I am so jealous! ^^ ~ Your English is very good! ^^ Well done! *^^* ~ To improve, make sure you use the correct words for the correct tense of your sentence. (For example: Yesterday was hot.) also, make sure to use plural words only when needed. (For example: Foods is not really a good word to use, as it doesn't sound natural!) ~ But your English is really good! I am amazed! ^^ ahaha ~ Good luck with studying English! ♥ ))