Tutor others in a subject you are familiar with. This may include tutoring a neighbor kid in a school subject, or teaching a local club how to make your signature potluck dish. The CELTA course instructs you in teaching and classroom management skills, but a dry run even before you get training may help you decide if teaching English to others is really something you want to be doing as a career, including temporarily.
Brush up on your understanding of grammar. Grammar is a dry subject and few teachers can teach it well; in fact, you might have had some substandard grammar instruction in your past. However, if you are going to be teaching it, you'll need a refresher on the subject. Of course, the CELTA course will have some grammar awareness exercises, but if your grasp of English grammar is rusty, it is better to slowly build up your knowledge of it before taking the course.
Start learning another language. The CELTA course instructs you in teaching English to adults for whom English is not their native language. The best ESL (English as a Second Language) teachers know exactly what it's like to study another language, including all the trials and frustrations. Adult learners can get especially frustrated when they don't know something or don't get something right on the first try. You should be able to empathize with them. In addition, as a number of jobs are located in non-English speaking countries, you should consider learning another language anyway!
Buy reference books. During the CELTA course and during your time as an ESL teacher, you will need reference books. Apart from any books the CELTA course may assign you, invest in a good English dictionary, grammar book and book of teaching ideas.