Strawberry   Strawberries  ??

Today I went to strawberry fields to pick (up) some strawberries. We could eat them (strawberries) for free! They were (It is) very delicious and I had a really great time. :) Love the winter vacation!

As your essay is about strawberries (rather than just one strawberry) it would be better to amend the title. We would say 'pick' if you went there to eat them. 'Pick up' is often used if you go shopping for bargains in a sale - hence the term 'pick up a bargain'. In your second sentence, you can use 'them' as you should try to avoid repetition. In your third sentence you changed tense from past to present, so just need to say 'they were very delicious'.