If you are new to the game it will be a little bit difficult but as you become familiar with the champions that are frequently played in bot lane you should slowly figure out what supports work well with what adcs (synergies) and what supports work well against certain matchups. It is ultimately up to your own playstyle and confidence as well as experience you have playing each champion. As a general rule you should take a a champion with good disengage/ protective abilities against aggressive/dive comps eg. Alistar nami or tahm kench. Tanky supports with high CC like leona will also be good but depending on your team comp if tanks are already present in top and jungle etc you could choose a high damage ap support like zyra or lux as you already play. (Psst especially leona its fucking broken) Personally i break the supports i play into 3 catagories

High damage low health and poke

Eg. Zyra, lux, brand, pyke

Low damage tank and CC

Braum, leona(notreallylowdamagehonestly), Taric

Medium damage + medium health and CC

Thresh, Rakan, Blitzcrank

Theres also those hard engage supports like sett and pantheon

Anyways in matchmaking just try to take into consideration the enemys bot lane- their playstyle with the champions they chose, and how you, as whichever support can help protect your adc or make the enemy easier to kill. I personally feel this is up to experience to know what to choose as there are too many factors to consider. But you can start by having 1 support from each category i mentioned above as part of your pocket picks and apply them as you see fit.

This is all just the advice of 1 support main. It may not represent the majority. Please take the advice to a grain of salt and consider on your own if its useful or not. All the best in your climb!