As the current beneficiary of the trust, you are entitled (in most states) to a copy of the trust agreement. This may not be true if you are merely a contingent beneficiary (i.e. you are only entitled to distributions from the trust after the occurrence of some event, such as the death of a current beneficiary).
First, you can request a copy from the grantor, if the grantor is still alive. The grantor is the person who established the trust. Second, you can request a copy from the current trustee.
The letter should establish the fact that you are a current beneficiary of the trust, and specifically request a copy of the trust agreement, along with any amendments.
In many states you will also be be entitled to an accounting, or a detailed set of financial statements detailing the holdings of the trust, and how trust funds are being distributed and spent.
If the trustee is unwilling to provide the requested information to you, you may need to speak with an attorney who can give you more specific advice regarding the law in your state.
You must determine whether you are a beneficiary or a contingent beneficiary. If you are a beneficiary, you are entitled to a copy of the trust and an accounting of the estate. If however, you are a contingent beneficiary, you are not necessarily entitled to anything because your inheritance will not vest until the contingency is met. You can also contact the "settlor" of the trust, who is the individual that created the trust. If you are a beneficiary, you may also contact the trustee and request a copy of the trust. If all else fails, contact an estate attorney and he/she will be able to assist you.