Remain positive and enthusiastic. Don't overdo it, though: watch your use of glowing adjectives (e.g. using "excellent..." too much can put employers off). You want to make a confident impression: find the balance between sounding over-confident (even arrogant) and under-selling yourself.
Create a professional application with your 'personal voice'. Experienced Human Resources professionals, who are likely to receive your application, can usually tell when you've based your application on a template.
You don't have to apologise if you have limited work experience: focus on what you would bring to the role.
A few additional things to think of when you create your cover letter:
- 'Less is more': don't use embellishments in the hopes your resume stands out, e.g. borders or shading (the content of your application is what counts)
- Your letter should be 1 A4 page max.
- Use plain, 11 or 12pt. font
- Make your documents easy to read, e.g. create appropriate, short paragraphs
- Importantly, proofread and proofread again: make sure there are no grammatical or stylistic errors.