This is a small guide on how to basically spend as little IP as possible while getting the most out of League of Legends. This won't really apply if you are not new.
* When you first start off in League, you generally want to collect the champions who are cheapest. This has the additional benefits in that the 450 IP champions generally are easier to play mechanics wise. These are the following champions: Amumu, Annie, Ashe, Garen, Kayle, Master Yi, Nunu, Poppy, Ryze, Singed, Sivir, Soraka and Warwick.
For new players there are 5 roles in league of legends, I will now split the champions in the different roles they can be played:
Top: Garen, Kayle, Master Yi, Poppy, Ryze, Singed, Warwick
Jungle: Nunu, Warwick, Amumu (If you collect runes which I will touch on in a minute some of the other champions can be jungled eventually)
Mid: Annie, Kayle, Ryze
Attack Damage Carry: Ashe, Sivir
Support: Soraka, Nunu and to a lesser extent, Poppy and Kayle.
* Once you have collected a few different champions it is now time to collect runes. You start off with two rune pages as default and this is usually plenty to make 2 universal pages(As an example I have been playing the last week or so with 2 generic rune pages to test how versatile runes are and they really are that good these 2 pages). Tier 2 runes cost 1 IP each(except quintessences) so a full Tier 2 runepage will set you back around 100 IP, and they give around half the benefit of a full rune page for a fraction of the price. Here are the two example generic rune pages:
* Refrain from buying the more expensive champions until you have played for a little while. There are several reason's for this. A) Most of these champions are hard to use for new players B) There is the risk you purchase a champion that is actually not as strong as other champions C) 9/10 times a new player buys a champion is because they saw an enemy player or teammate wrecking face with it and literally feel like throwing money at the screen to be that good. The difference is that just because it works for them might not fit your playstyle or you are not experienced enough to use that champion effectively. D) It is much more efficient to save IP for runes and the very occasional times you see IP sales for champions(or even skins!).
* If you follow the above advice you should have about 10k IP at least by the time you get to level 20. Now you can upgrade your rune pages to Tier 3.
* Once you have 2 full Tier 3 rune pages at level 30, and the above champions you have a good base to build on. You should have around 13 champions and in order to play Ranked you will need 3 more. At this point your IP can be spent as you like, either purchase more rune pages, more champions, or simply save it.
* This is a bit of an obscure tip but I'm going to recommend champions that are good buys because they are usually strong, safe picks that can be played in various ways and so are almost like 2 champions in one.
Ahri, Blitzcrank, Caitlyn, Darius, Ekko, Elise, Ezreal, Janna, Jax, Jayce, Karma, Kennen, Kha'Zix, Le Blanc, Lee Sin, Lucian, Lulu, Lux, Maokai, Morgana, Orianna, Renekton, Riven, Rumble, Sona, Syndra, Thresh, Zac and Zed.
Please note that some of these champions have some complex mexhanics.