There are many champions that are easy to learn by simply watching a good player use them. Watch what abilities/items they use, and try to ‘copy’ it. While you won’t necessarily have the split-second timing and reflexes the pro had, generally the fundamentals are sound and unless you are up against a pro yourself the opponent will be just as susceptible to a novice.
Some champions are more complicated than others. On Howling Abyss, you have a steeper learning curve because you don’t have the serenity of killing creeps safely and only having to worry about 1 or 2 opponents bothering you. Personally I find champs with ‘binary’ abilities the trickiest to learn (Each of their abilities has an ‘alternate’ ability that gets used under some condition, like being in ‘cat form’ or hitting the opponent with some other ability’).
General, simple strategies for Howling Abyss:
Win together, or die alone: Remember that all 5 of your opponents tend to be clumped together. Get a little too close, and they are all going to gang up on you. If they have multiple stuns, you are pretty much screwed as soon as one gets into range. Be smart and work with your teammates! The lone rambo bulldozing through everybody only works if he has a significant gold/level/ability/skill advantage. In most cases trying this will give the other team 1 kill and 4 assists.
Wait up!: If you’ve just destroyed a tower, it can be tempting to rush forward and press your advantage. But if you had a few teammates die, and the rest of you are at 1/3 health, hang back and let your buddies catch up to you; recently respawned opponents will be at full health and likely outnumber you. If you get too agressive, they’ll just wipe you out ‘aced’ and kill your tower in retaliation, ruining your advantage.
Itemize for teamfights: Many champions in LoL are ‘junglers’ ‘assasins’ etc. Designed for a particular role that doesn’t necessarily lend well to a 5v5 25 min long teamfight. Typically single-target melee champions have a much harder time on this map, but they’re not impossible to use if you are smart. Prioritize your abilities and items for a teamfight- if everybody else is squishy, build yourself up tanky. Items that give ‘aura’ abilities, health regen and mana regen are hugely useful.
They’re not OP, learn to play!: Some champions seem blatantly overpowered on this map. Thats because they might have a lot of AoE abilities, hard-to-miss skillshots, or good sustainability. You have to learn to counter them. Katarina has an ultimate that can flatten 3 enemy champions pretty quick, but she can be stunned out of it. Similarly, Fiddlesticks has a devastating ultimate that can net easy pentakills against poor opponents, but you can also stun him out of it or stack magic resistance.