I'm going to translate Elda's reply (since Monica is a beginner, and I found Elda's reply helpful in mentioning some things I didn't know before.)
1) Mi amiga Kathy es delgada pero yo estoy con sobrepeso.
Here you need to put the pronoun "yo" (first person singular) as the subject, in order to differentiate the subject of the second clause from the subject of the first. Also the preposition "con" is needed to link the verb ("estoy") with the noun ("sobrepeso"). It is a compound sentence, with two distinct subjects and verbs, united by a conjunction.
2) Mi amiga Sarah tiene el pelo corto y yo tengo el cabello largo.
As before, you need to put the subject in the second clause because it refers to a distinct person from the one alluded to in the first clause. That's to say, in the first clause, the subject is "mi amiga Sarah," but in the second sentence, the subject is "yo." They are distinct persons, so it is mandatory to mention the subject in the second clause so that the construction will be well formed and the concept understood clearly.
3) The Spanish equivalent to the English "outside" is "al aire libre." The Spanish equivalent to the English term "inside", in this context, could be well put as "en recintos cerrados" (with which we are indicating gyms, courts, etc., -- in other words not a specific sport, so the term is general).
4) Me gustaría saber cómo cocinar muchos platos, pero mi amiga Erin sólo sabe preparar queso a la parrilla.
I'll explain it: in Argentina, we call it cooking "al gratén" when you put cheese on top of a dish and put it in the oven briefly so that the cheese will melt. Nevertheless, the RAE dictionary lists the verb "gratinar" with the meaning of lightly toasting the surface of a food in the oven. In cuisine and catering, there are many variants, according to culture and region. En Argentina, what is called "grilled cheese" in English, we call "provoleta." It is a round portion of cheese that is usually seasoned with oregano. It is a customary/traditional part of (a barbecue') in Argentina.
I hope all of this is of help. Greetings, Elda.