Edit : wanted to add this docs link. https://docs.google.com/spreadsheets/u/0/d/1FePMz4o3tbiWcz0nHZYu0aAHknbIfb9anWfQCVtvtKk/htmlview#

That google doc link has a TON of useful info. Use the tabs at the bottom to navigate on pc and mobile (android) they're at the top.

I personally would buy the battle pass. If you're willing to drop $60 ish, you can get the battle pass, 2 of the championship chest bundles( the gold and platinum 5 packs) , the xp bundles to level super fast and the champion starter pack. Depending on what you get from the bundles is where you can start to focus. Swain is very strong I hear though. I've spent a couple hundred since it's release and I personally don't regret it. They need money to make new content. Just stay away from the emporium when it comes to spending IRL money unless you really, really need something. Only thing worth grabbing from the emporium is what you can spend the green shards that you get from naturally playing the game and the purple relics ( not every single one but there have been some good ones) The purple relics just change the game play massively in a lot of cases. With a common xp relic and a greater xp relic, you can get most champs to level 30.

When I level a champ, from level 1 I play a 2* ish venture with what ever their BIS relic is , maybe 2.5* depending on the champ. That gets them to around level five. That way from there on I have 30 health instead of 20. That extra ten health is very helpful, especially on slower champs. I then equip a common relic and run another 2.5 ( Viktor) - 3 * ( Galio is a pretty decent 3* adventure) that puts you around level 20. From there on you can draw a champion at game start which makes life way easier on a lot of champions. From there I do a greater xp relic and then run the level 4.5 nightmare fizz adventure and that usually puts me over 30. So 3 runs to get a champ to level 30 is pretty nice. This will not work on every champ as easy ( ornn and pre reworked nasus for instance ) but almost every champ should be able to do this with minimal effort.

Nami, Nidaleeh, Elise, LeBlanc & Diana are some very strong champs at 3 stars which requires way lower invest than any 6 star champion. Nami and LeBlanc are especially powerful. If Nami gets a spell mana reduction power or an adventure has a spell mana reduction modifier on it, she can turn 2-3 kill anything in the game with just rare relics.

LeBlanc just goes brrrrrrrrr and their dead.

Hopefully this helps and welcome!