On a cover letter mention:
-Name, age, school you're from
-Dates you want to go
-Why you want to go
-What you hope to see/do (watching consults, procedures etc)
-Briefly mention previous experience
As you're including a CV a cover letter doesn't need to have loads of detail. Make sure you make it persona,l though, phone up the practice and ask who you should write to. I'd suggest you do this ASAP as some practices get booked up with work experience quite far in advance.
Just to add, it's great that you're organising a placement at a vet practice, but if you're serious about applying for veterinary medicine then you'll need to gain a good deal more experience in various places including large animal practice, farms (dairy, lambing etc), stables and kennels. Some universities place more weight on work experience than others but many applicants have 15-20+ weeks (remember that a lot of people end up taking gap years and reapplying so you'll be up against them). Check out the 'Work Experience Bible' at the top of the forum for more information.