As to both your questions: the squad leader can mark an objective to attack/defend with the middle mouse button while looking at the objective marker. If you're too close, you won't be able to mark it, so try backing up a bit. This also prevents the focus from automatically switching to the next unowned point.
As for general tips:
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Don't forget to put down your rally beacon whenever you get enough squad points. Find a relatively intact building or some natural cover to put it behind and maybe shore it up with some sandbags. Put it somewhere either close-ish to a point or at least far forward from spawn.
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Do remember to re-evaluate which point you've ordered to attack/defend often; your squad engaging at these places is what get your squad points you all use for building.
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If a lot of your squad is dead, try to find cover and disengage from combat so they can all spawn in on you.