Standing is the Jewish form of prayer. Why? One explanation is that we approaching G-d as our king. Just as you would stand in respect before a human king (or president or whatever other important person), we stand before our heavenly king. I’ve also read somewhere (cannot seem to find it again) that lying down would be too comfortable, kneeling would be inconceivable as we are created in G-d’s image and G-d would never kneel, so that leaves standing and sitting which are both positions of Jewish prayer.
When do we stand? One important rule is to stand whenever the ark with the Torah scrolls inside is opened or the Torah scroll itself is in motion (e.g., it is carried through the synagogue). This should be done by everybody, no matter what he/she may be doing at the time. Other important prayer parts where if possible everybody should stand are Barchu (the start blessing of the morning or evening prayer) and Kedusha (insertion during the repetition of the Amidah).
There are a few other prayers where people usually stand, these are the main ones (that I can think of right now):
- Amidah (the main prayer): At least stand during the silent part until you have finished. Then you can sit down and wait for the others. Get up again when the leader starts the repetition, you can sit down again after the "Kedusha".
- Aleinu leShabeach (final prayer)
- Baruch sheAmar (first prayer of Pesukei deZimra on morning)
- Yishtabach (last prayer of Pesukei deZimra on morning)
- Kaddish (prayer for the dead, among other things): In Ashkenazi communities. In Sefardi communties you keep the position you had, so if you were standing you don’t sit down, if you were sitting you don’t get up.
- The last verse of Lecha Dodi (Shabat song): You also turn around to face the door.
- Kiddush and Havdala (blessings for start and end of Shabat and holidays): There are different traditions, but at least some communities stand.
- Psalm 29 (during Kabbalat Shabat): Again, may vary.
- VeShamru (reminder of shabat): May vary even between standing on Friday evening and sitting on Saturday morning.
Good siddurim (prayer books) will indicate when to stand and when to sit. Sometimes there are regional variations, just go with the flow, if everybody stands, stand as well. If everybody sits, sit down. The important part is to seem natural. If you suddenly notice you are the only one standing, don’t look around in panic and sit down quickly. Sit down as if it has been your plan all along to stand until now and sit down at this exact moment. Most people will assume you know what you are doing and you have a different tradition that requires standing until this point.
What if you cannot stand or cannot stand for so long? Sit down!!