Priority number 1: WARD
Keep a ward up in the river at all times. Even if you have to back to base to buy a ward, do so.
Not warding your lane is asking for a double kill at bot and a surrendered tower. It usually also costs you dragon. Therefore, a 75 gold ward can prevent a 2500 gold swing in favor of your opponent.
I prefer vision wards at dragon to destroy their vision and help your jungler. The best place to put sight wards is on top of dragon essentially. The best place for vision wards are in the bush by lane. This lets you see most sight wards that they placed whether it was in the bush or near dragon. If you vision ward outside the bush, you cant see any sight wards inside of it, so that is why the placement is different.

Priority 2: WIN LANE
Your main focus is to keep the carry alive. Depending on who is playing, you may need to prioritize heals over infuse which gives them mana. The easiest way to harass as Soraka is to let the carry have unlimited mana to harass for you. If you are playing with a champion that can abuse mana spamming such as Corki, Urgot,Miss Fortune, Caitlyn... etc try to get level 3 infuse quickly. If mana won't really help them harass(such as a Vayne) then just get heals to keep them in lane longer. If you get starcall try to prevent lane fights by infusing their carry and starcalling when they go in. It should be enough damage to prevent them from committing. If they still commit, spam starcall and heals and it should go your way, since you do respectable damage, especially in a drawn out fight.