Your clarinet will come with a reed and cork grease. At your first lesson your teacher will show you how to correctly assemble your clarinet, and they will put the reed in place. The reed is delicate and once split or damaged will no longer be useful.
It is essential to have some spare reeds. They come in different strengths notated by numbers (i.e. 1.5 / 2 / 2.5). Beginners should opt for a 1.5 strength reed. When you buy a reed, make sure it is for a B Flat clarinet. Reeds are relatively inexpensive and should be available from all good music shops. If ordering online then you may have to buy in bulk (packs of 10 or above), but this can prove cheaper in the long-run.
Hygiene. It is important to keep your clarinet clean. You will need to purchase/make a pull-through to clean the moisture from you clarinet. At the end of each practise session, use a clarinet pull-through to ensure the inside is clean and dry – your teacher will help you do this. Use cork grease occasionally to keep the different sections fitting well.
Your instrument is yours to look after – do not allow anyone else to play your clarinet! Keep your clarinet in the case when you are not playing it.