HI Sweetie,
Can your grandmother still answer yes and no questions? If so, ask her if she'd like some lotion rubbed on her feet, legs, hands and arms. This will feel good to her if she says yes. Maybe you can paint her nails as well, just because it's a way for you to be close to her. Keep talking to her about anything and everything. She loves to hear your voice. If you notice something that she might need, speak up on her behalf. This disease sucks big time, and the little things that we notice makes all the difference.
The man I took care of was named Andy. He had two teens. They made sure to bring in a cd player and played his favorite music for him. He enjoyed it very much because he really didn't feel well enough to watch tv. A friend came by every evening to read him the Bible, some favorite passages. We all would have "normal" conversations in his room as well, so he didn't feel lonely when he was awake. If your grandma has been missing church, upload the sermon and play it for her.
I guess anything you can do make her feel comfortable and loved. You know her best.
Be sure to give yourself a break every day and go for a walk. Is there a good friend you can talk to? A favorite teacher you can lean on? Don't be afraid to just go and say, "I'm having a hard time with this." She loves you sooo much, and I bet she is worried about you dealing with all this.
Hang in there. We are all here for you. Also, check this site out:
http://www.als411.ca/ It's a support group for teens like you.
Blessings,
Pam