Both are correct but mean something different. First means maybe I’ll be free but I don’t know. If so, I’ll go with you. Second means I’m not free,but if I were I would go.

Both statements are grammatically correct but they apply to different situations. Read Phil's excellent article.

Both are correct.

You can find a short summary of all you need to know in my brief mini-article: Grammar boost: Conditionals cheat sheet PART 1 https://www.italki.com/discussion/164505

The second sentence: "If I were free, I would go with you". The first one would be correct without "if": "I am free, I will go with you."