Strictly speaking, the idea behind the concept of language exchange is not that you teach! Doing a language exchange assumes—in your case—that you have at least a basic knowledge of Arabic and that your partner has at least a basic knowledge of Spanish. A typical way of doing a language exchange is that you speak the language of one of the partners in the first half of the session, and the language of the other partner in the second half of the session, e.g. one hour in Spanish and one hour in Arabic. Most people are not trained to teach or to explain the grammar rules of their native language, so trying that may lead to inaccurate input.
If both partners have a different level in their target language, the topics and possibly the type of interaction may differ considerably depending on the target language that is being practised. You need to use your imagination. For example, you can agree to use the FORCE cycle to practice oral skills, which require some preparation in advance of the session. You can also use conversation cards if you run out of topics to talk about (see the bottom of the article Language Exchange – So What Do You Want to Talk About?.)
If one of you does not have a basic knowledge of the target language and you want or need to go down the "teaching" road, have a look at Total Physical Response (TPR. (It may be a bit artificial at the beginning, but it's a start. You can find many videos about it on YouTube.) If you try to teach your language without being trained as a teacher, also consider these tips for lay teachers.