Soraka is a support champ meaning that you should not go solo lane every game. She has the best heal in the game with Astral Blessing. She can also silence champions or restore mana to your allied team mates with Infuse and it costs no mana at all. Her Q Starcall is her offensive/support ability which deal damage and reduces enemy champs magic resistance and it can stack up to 10 times and is on a short cool down of 2.5 sec. Soraka can lane with anybody and help them against all champions.

greater quintessence of knowledge gives you extra mana for late game for that extra support sustain.
Greater Seal of Armor gives 11 armor which should help you in your lane against physical damage.
Greater Glyph of Scaling Ability Power gives decent amount of AP for late game damage with Starcall.
Greater Mark of Magic Penetration gives magic pen making your abilities more useful.

The masteries make you more of a support giving you extra g/s and more useful.

I go with an unorthodox starting item with 2 Faerie Charm making you spam your abilities or heal your teammate more frequent. sight ward for a safer lane and a Health Potion instead of mana cas you should keep a safe distance and heal your teammate instead of wasting your Astral Blessing on yourself.
First trip back should get you a philosopher's stone and Boots giving you more g/s.
Tear of the Goddess is a great item since its unique passive is so beast and getting you that 1000 free mana should be a breeze with your Starcall short cool down. It takes 250 ability casts to get that 1000 free mana when calculated with just Starcall alone it would take 625seconds or 10 and a 1/2 minutes but since it can't be done more than twice every 6 seconds than 250 ability cast * 3 seconds= 750sec or 12.5 min thats if you do use it every 3 secs but you should have it but 30mins.
Boots of Swiftness is my fav on any champ cad there nothing more frustrating than being slow.
Shurelya's Battlesong gives cooldown, health, mana regen and a nice active that can help you teammate get a kill or escape death.
Archangel's Staff has a nice passive too which is 3% of your mana is AP so all that mana is not for TROLLING. With this passive alone you should have 88.35 AP boost so in total your AP would be 88+45+75=208AP thats when you get all the items and 1000 free mana.

Soul Shroud and Aegis of the Legion both have UNIQUE Aura making your team stronger as a whole and team fights more against your favor. With these items you should not die easily and neither should your teammates.
Morello's Evil Tome last item makes your spells stronger and its active is very useful and gives you an advantage as well.
These items complement each other well and focus more on SPAMMING your abilities while serving a purpose for your SPAMS.

Astral Blessing should be your primary spell and will take you to victory making you a great team player.
Infuse is second since it has no cost at all and it does great damage and has a silence.

Leave Starcall at lvl-1 since you are just using it for the magic resist debuff.
With soraka you should always keep on eye on the map and your aliies health bar at top left. Make sure that if they are fighting and you don't think they will come out alive pop Wish which gives health and some time for your ally to get the kill or worst case scenario retreat.

Clairvoyance revels a small area of the map for a few seconds, use it to check baron, dragon, brushes, and blue&red buffs if they have a jungler.
Promote i think is helpful pushing lane giving you some offense instead of tower hugging if that becomes the case.
Heal isn't a bad choice either
Ignite, Ghost, Flash, Exhaust are also acceptable

In lane all you should do is not try to last hit all the time but to try to Infuse the enemy champs, Astral Blessing your teammate if needed and just keep moving to keep the enemy guessing. Don't heal your self all the time unless you are at 1% health even then think about healing your teammate instead.

Don't be the 1st one in rather stay back and try to Infuse the mage Starcall as much enemy champs as possible and Astral Blessing the sh*t out of your team mates. Wait till the health drops below 50% to pop Wish other wise the other team can flee too early.

As a support your job is simple but very crucial to your teammates. You might think being a support is lame but a good support should stack up mad assists and a few kills with little to no deaths. even if you don't get a kill it is not a problem nobody will get mad at you. You should be the one responsible for map awareness and should ward key areas of the map.