As the steps may vary depending on your phone model, please refer to your phone user manual for details. 

Find the contact in your Phone Book by selecting Menu → Contacts → Find Name. Highlight the contact you want and press OK. Then select OPTIONS and press OK. Choose SPEED DIALING and a number will be assigned to the contact you chose. Press OK to assign that number and you will now be able to call the contact by press-hold that one number instead of 10 digits.