The first two sentences should encapsulate the problems (that you want them to pay you to solve) typically experienced by potential clients. This will show the client that you know and understand their business/industry.
The rest of the (short) first paragraph should enlarge on the first two sentences.
This is not what you can do for them but rather what needs to be done.
Briefly how you have achieved solutions for past and existing clients. Name a few but leave out their direct competitors.
Your contact details will be on your letterhead but it may be an idea to say:
If you feel I can be of assistance to you please call me on xxx-xxxx or email me at: firstname.lastname@example.org
- Get your own domain. Using Gmail makes you look like an amateur
- The above also applies if you are using email to reach potential customers.