Here are your options.
- Choose a support to cover your ADC's weakness.
- Choose a support to cover your team's weakness. (No escape, provide it.)
- Choose somebody with CC, if your team has none.
- Don't choose champs if you can't land abilities. (Unless you are practicing.)
- If you are good when certain supports, counter shouldn't matter. Everything is skill matchup.
- If you rely on counter picks, learn why they counter them.
- Choose someone that synergize with your ADC. (Provide combos. Some champs can chain CCs or output tons of damage while CC'ed.)
Items.
- Buy what you need, but still ward. (Even if you have to buy wards.)
- Boots if you are out of lane a lot.
- Sightstone if you are pushed a lot. (or if mid is getting ganked a lot for bottom jungle.)
- Items to cover weakness.
--- Team full AD and have an AP champ? Cover the AP side and build AP.
--- Team need a Tank, build tanky.
--- Team need more utility, get the items.
- Items are situational for supports.
- I usually rush Tier 2 support item for gold income.(You don't have to do this.)
Nami and Thresh aren't the only supports out there, but for your first question.
Nami
- Team Peel
- CC
- AP or Utility (Can go slight tanky.)
- Heal
- Speed Boost
- Chase Kill (Careful with chasing. Chase only into wards. Objectives more important than kills.)
Thresh
- Engage
- Rescue
- Individual Peel
- CC
- Tanky or Utility (Can go AD or AP.)
- Shield
Also, if your team needs exhaust grab it. Most times the team prefer you go exhaust than ignite.
That's all I got so far. I'm sure I miss something though.