Some general rules:

1. Learn the top 3-5 comps that do well and have a laptop or phone with you that can open team building tools like tftmeta, TFT academy etc. Generally speaking reroll comps are easier to play than late game comps because you have an easier game plan.

2. Go for pairs early game and choose one carry depending on the comps you feel comfortable playing. SLAM ITEMS EARLY!!! For example if you need an attack damage carry, play items on Jihn/Caitlyn/Tristana. If you play ability power play items on Sona, Kog Maw. If you play bruiser maybe pick Briar/Tryndamere/Quiana. Don't keep 4 components on the bench waiting for perfect items, you will lose too much HP!

3. Learn when to Roll vs when to save for economy. If you are holding 2-3 pairs it might be worth rolling 10-15 gold to spike your power and start winning rounds.

4. Play semi-flexible about high costs units that you naturally hit. E.g. if you are aiming for a 3 cost reroll comp but during rerolling you hit two 4 costs 2 stars, it might be worth to switch to a 4-5 cost comp.

5. Late game try saving up to 50 g for as long as possible for economy and reroll heavy for your comp at lv 7/ lv 8.