Why does it say not to fear?
Radak: Do not fear other gods. Also Yirmeyah said not to fear other gods - they do not harm or help.
Malbim: Deniers say that I cannot override nature; My power is limited. [You know that this is false, so] you need not fear.
What is the meaning of "v'Al Tirhu"?
Rashi: There is no word like this in Tanach. From context, it means do not fear [to inform My name among Nochrim].
Radak citing Targum Yonasan: Do not be broken. This is like "Lo Sira v'Lo Sechas" (Devarim 31:8).
From when did Hashem make you hear?
Rashi: From [Ma'amad] Har Sinai, I told you that there is no god other than Me.
Radak: From when I told you future events, and they came.
How are you witnesses about Hashem?
Rashi: He opened the seven firmaments, and showed you there is no other [god in Shamayim]. You can testify about this!
Radak: I sent My Nevi'im to you, and did miracles and wonders with you. You know whether there is another G-d like Me - "Oh ha'Nisah Elokim Lavo Lakachas Lo Goy mi'Kerev Goy b'Masos b'Osos uv'Mofesim
What is the meaning of "v'Ein Tzur Bal Yadati"?
Rashi citing Targum Yonasan: No one has strength, unless I gave strength to him. This is like "v'Eda'acha b'Shem" (Shemos 33:17), "Yada Lechtecha" (Devarim 2:7).
Radak: There is no rock (power) that I do not know, and they do not truly know Me, for they are not G-d. This refers to the angels and spheres.