You've not stated how you're sending your mail. Is it via a website or through an email client? (If so, which one? Outlook? Outlook Express? Windows Mail? Thunderbird?)
What you need to do is create a group containing all of the relevant addresses. In Outlook Express that's achieved by opening the Address Book and selecting New>Group. Then you can select the addresses you require.
To send mail to everyone in the group, simply put the group name into the 'To' field. If you don't want everyone in the group to see each other's addresses, put your own address in the 'To' field and the group name into the 'Bcc' field.
Open Outlook, go to File, New, Distribution List. Name it, then add any of your contacts by going to Select members, or add new ones. Save and close. The list will appear in your address book and when you need to send an email, just click on it to send it to your email.