Soraka lanes well with and against AP champions (casters); Consecration and Infuse will allow you to have more control over your lane against them and Starcall's debuff will help increase the damage of your AP partner and Infuse will help mana constraints.
Leveling up Starcall early may limit your utility, but it can help you harass better than many other champions in the game and it lets you farm up money for items more easily.
You should level up Starcall when your team is completely in control of the lane and you want to keep the pressure on, or want to farm up a lot of gold.
Soraka is extremely strong in the laning phase. Getting one point in Starcall while laning allows for powerful harassment, denying creep farming. Using Infuse to harass allows for a strong push. Since she can heal herself, playing aggressively can be rewarding, as long as you are not being overly reckless.
Starcall uses very little mana and has a short cooldown, which makes it effective for farming. It can only be cast when a target is in range. If there is a stealthed enemy (whether by ability or by being hidden in brush) within range they can be targeted.
Don't forget that Starcall reduces enemy magic resist. The stacking 8 to 12 reduction per cast will make a difference if your team has magic damage, but don't risk your life to get in Starcall's short range. The magic reduction also increases your damage if you need to fight.
Keep the magic resistance reduction in mind because you never know when getting in a Starcall before Infuse rather than after will just barely give you enough bonus damage to score a kill.
Leveling up Astral BlessingAstral Blessing early will be the most taxing option for Soraka as far as mana consumption goes, but after a few levels, it allows her and her teammate to shrug off all but the most tenacious harassment.
You should level up Astral BlessingAstral Blessing when you and/or your teammate constantly find yourself at low health, or are constantly being harassed but you aren't having mana problems.
On the off-chance that you find yourself leading a solo push against a tower, consider using Astral BlessingAstral Blessing on a Siege Minion. These minions take half damage from towers, so healing it and buffing its armor can increase its longevity by a substantial amount.
Leveling up Infuse early is probably the most balanced aproach. Infuse has both offensive and defensive uses for Soraka.
You should level up Infuse if your teammate is constantly running low on mana or if you are laning with a champion who would appreciate the ability to cast spells constantly such as Brand.
Late game, Infuse should be used on enemies with channeling spells that deal a great amount of damage such as 卡特琳娜的Death Lotus for the silence rather than restoring allies' mana.
Using Infuse against champions that rely on spell combos or escape abilities is very effective. For example, Annie relies on a stun and burst. A well-placed silence between will let the stun victim escape without harm. This can also work "aggressively", silencing champions with strong escape abilities such as Vladimir, LeBlanc, Kayle, and Poppy.
You can cast Wish on your allies from across the map to save them from otherwise fatal events. Keep an eye on their health bars on the left side of the screen and cast when appropriate. This is also an effective way to grab assists, even if your allies aren't in absolute danger.
If you are playing an arranged team, Soraka can be extremely useful due to her ultimate.
It may also be pointed out that building Soraka with high amounts of ability power can make her an extremely aggressive champion.
Keep in mind not to get too greedy as Soraka's abilities do have a somewhat low range.
Be careful when playing against champions that can reduce your healing ability, such as Miss Fortune, Fizz, Tristana or Katarina. Similarly, watch for enemy support champions with Morello's Evil Tome, as the active ability (50% healing reduction) will greatly reduce your heals on allies.
Due to how spammable Starcall is, and the high AP ratios on her heals, Archangel's Staff can be a great item to get early. The extra AP translates into extra survivability due to her heals.
Getting a Rylai's Crystal Scepter not only is a great way to get AP and HP for Soraka, it also allows her to constantly keep the entire enemy team moving slowly with Starcall and/or slow a single opponent to a crawl with Infuse.
The combination of slow from Rylai's Crystal Scepter and silence from Infuse is a great way to prevent a champion from escaping while your teammates kill him.
One option for Soraka in especially long games is Lich Bane since all the stats are very useful on her and she can use Starcall to proc the on-hit effect to make her more potent in a team fight.
By late game, Soraka's heals are so powerful, they can render much of her team nearly invincible. When this happens, the other team may start focusing Soraka, so grabbing an item like Guardian Angel or Zhonya's Hourglass can keep you alive long enough for your heals to come off cooldown.
Most of Soraka's base spell power and AP ratios are directed towards healing her teammates rather than damaging enemies. This makes Ionian Boots of Lucidity or Boots of Swiftness her "offensive" boots of choice over Sorcerer's Shoes, since they increase her ability to close into spell range and/or spam once there.
Starting off with Boots of Speed, one Sight Ward and one Health Potion is one opportunity which gives Soraka the invaluable ability to place a ward close to her lane in the early laning phase, which can save her and her partner from early jungler's ganks. It also increases survivability by increasing movement speed.
Starcall is spammable and reduces your magic resistance whenever it hits you. Either try to stay away from her outright during laning phase so she won't be able to harass you, or wait for the magic resist debuff to fade before closing in again.
Always have an Ignite or Executioner's Calling ready whenever you are going to engage an enemy that have Soraka in their team in order to reduce the effects of her two healing abilities.