To begin organizing. yourself to accomplish all your tasks you need to

you need to have a plan to do so

have a plan and a clear mind

with whom will you be coming to the party? this is written correctly.

the pronoun reference replaces a noun, but you have to make sure there's no room for confusion.

in the first sentence "her" could be referring to shely or to the sister. the answer there is "shelly called her sister and told her about the new baby."

in the second sentence "it" could be referring to the roof or the floor. the answer there is "since the roof leaks on the family room floor, we need to repair the roof."