Best strategy on buying champions and runes in LoL?

When buying stuff keep one thing in mind: You cannot buy runes with riot points! This is pretty heavy since runes are amongst the most important things in LoL at lvl 30.

TL;DR

• Buy runes at level 20 for AP/AD/Tank and only buy champions with IP after you have at least 2 Rune pages.
• Buy a champion bundle for RP.
• If you can afford it buy RP until you have about 80 champions and always keep an eye on sales.
• Don't Waste money on Boosts.
• If you know a champion already It's worth buying it of course and If you're interested in one just buy it aswell. If you're not sure about whether or not you will like the champion don't buy it immediately (watch some playstyle videos first)
• Play at least once a day to maximize your IP gain (FWOTD bonus)

A bit more detailed answer

You basically want to save up for at least 2 runepages which will greatly increase your chance of winning. For me personally I reached platinum Elo with only 3 rune pages so It's really an investment that's worth it. You should start buying runes at level 20.

When it comes to buying champions you should probably buy a Champion bundle. In a bundle you will have 20 champions and a skin for the price of roughly 5 champions. The problem is that you have to spend real money for bundles.

If you don't want to spend real money I'd suggest to buy runes first and then champions you know well. Also the price of a champion doesn't determine it's strenght. Some 450 IP champions are just as good as those who cost 6300 IP (if not better).

You shouldn't be spending RP on boosts unless you want to play for 3-30 days straight. You will get enough IP to afford stuff with it. And once you have enough games you will notice that IP gains are pretty high (I got about 300k IP right now and I have no Idea what to do with it).

The Most Efficient Use of IP/RP

Runes require IP but champions can be bought with IP or RP. The earlier champions are relatively cheap IP/RP, but the later ones are 6300 IP, a large amount of IP that takes a long time to acquire. Luckily, 3 champions are on sale every 3 days, cutting the RP but not IP price.

Since runes can only be bought with IP, spend your IP on the Greater runes starting at level 20 and use RP to buy champions from the most expensive tiers (>3150 IP) that are on sale.

If money is no object, you can buy a large pool of champions in only a few weeks. The cheap champions can also be bought with RP, but they are cheaper than some of the Quintessences, so just buy those with IP to make your dollars/RP last longer. If you want to further stretch your dollar, research champions that are not meta friendly or have low win percentages and skip buying those, prioritizing the champs with high win rates, ban rates, or are common picks. (use lolking.net)

Once you've bought general use runes for most champs (armor seals, damage marks, magic resist blues, etc.) start buying more champions with IP.

If you are new to the game, you will undoubtedly buy champions that you don't like or are not skilled using when following this method. If you really want to become skilled at this game and climb the ranked ladder, you will have to become proficient and familiar with a large champion pool anyways, so it will be worth it in the long run.

The Digital collector's pack, Starter pack, and Champions bundle are worth the RP, but include many cheap champions that could be bought with IP.

If you want cheap champions fast, buy the bundles. The Champions bundle is especially worthwhile because it includes more expensive champions, while the starter pack includes many cheap ones. I'd personally buy the Starter champions with IP and use RP for the other packs to stretch my RP for sales.

Use RP to buy a rune page bundle.

Rune pages are not cheap at 6300 IP. This is really the only place where Riot forces us to spend real cash on their game, otherwise it's rough saving up the IP. This just makes it easier on you and accelerates your learning of runes as you have more room for experimentation.