When you are being attacked by other players your fellow alliance members can be a big help to you. However, unless your alliance members are constantly checking the attack log at your alliance profile, first you will need to inform your allies that you are under attack.
You can either do this by messaging your allies directly, or you could ask your alliance leader for help - they can send out a message to all of your alliance members so that you can get all of the help available. Your alliance may also have a forum where you can post asking for help (you should know if your alliance has a forum for this).
Whatever way you choose to ask for help it is essential that you sent the following information to your alliance colleagues:

• Attacked village name(s)
• Name of your attacke(s)
• When do the attack(s) arrive?
• If possible: which kind of troops are in the attack (e.g. catapults)?

Once your fellow alliance members know that you require help they should send reinforcements and resources to your village. Because the reinforcing troops in your village will need crop to survive you may need to upgrade your granary, so that you can hold crop for the troops to eat. It is also a good idea to upgrade your wall, as that will make your reinforcements more effective while defending.

After the attacks against your village have come to a halt you can return your alliance members' troops back to their owners from your rally point.