Can I attack with the Spiritual Weapon spell as an action, and how do I determine the target?

Question

1. The description of the Spiritual Weapon spell says:

   ... When you cast the spell, you can make a melee spell attack against a creature within 5 feet of the weapon. ...

   Does it mean I can attack with Spiritual Weapon with my action only in that case?

2. If it doesn't, then does it mean I can make 2 attacks with Spiritual Weapon using my action and bonus action next turn?

3. It's written:

   ... As a bonus action on your turn, you can move the weapon up to 20 feet and repeat the attack against a creature within 5 feet of it. ...

   Does it mean I should choose only 1 target for Spiritual Weapon?

Answer 1

Contrary to what you imply in the question, spiritual weapon takes a bonus action to cast (PHB 278), and thus you cannot attack with it as an Action even at the time of casting. About the later attacks we read that:

As a bonus action on your turn, you can move the weapon up to 20 feet and repeat the attack against a creature within 5 feet of it

The spell does not grant you the option to use your action, only your bonus action. Since you have only 1 bonus action in a round, you can only attack once and only one target ("attack against a creature" = 1 creature).

However, there is no indication that you have to attack the same creature every time, so you can choose each time you attack.

Answer 2

It's darn near impossible but…

Technically, yes…

The initial melee spell attack is part of successfully casting the spell, however that occurs. If you manage to cast spiritual weapon as an action, such as through the use of an artifact with the minor beneficial property (on a result of 71-80) of allowing you to use your action to cast a particular 2nd level spell as chosen by the DM, a bead created by the Arcane Abeyance feature of a Chronurgy wizard who somehow has spiritual weapon, or a similar magic item, leaving your bonus action free to make use of it again on the same turn, you could potentially attack with it twice in one turn.

However…

The spell itself doesn't allow you to use your action to cast it, or to to repeat the initial melee spell attack with it. You can only use a bonus action on your turn to repeat the initial melee spell attack, and usually you will have already used your bonus action to cast it on the turn it appears, because it has a casting time of "1 bonus action".

So…

1. Only in the extremely rare case that you manage to cast spiritual weapon using your action, could you make an attack with it as part of that action. This will almost certainly never be the case.

2. It does mean that, though heavily caveated. So no, you cannot make two attacks with spiritual weapon on your next turn.

... As a bonus action on your turn, you can move the weapon up to 20 feet and repeat the attack against a creature within 5 feet of it. ...

You can only use a bonus action to move the weapon up to 20 feet and repeat the attack.

3. When you repeat the attack, you make a melee spell attack against a creature within 5 feet of the weapon. It doesn't have to be the same target each time.