Beginner Initiators to Learn

I have noticed the lack of both supports and initiators in pubs at my level. The lack of supports, while a big problem, is usually balanced since BOTH teams lack supports. However I feel that lack of initiation is a bigger problem. During "teamfights"(if these engagements even count as teamfights), there is no one to start it so both teams usually wind up dancing around on their own side of the river while killing the creep wave that walks up to them. This dancing continues until some person walks up with a squishy hero, gets kills, and that team winds up losing the "teamfight". Because of this, I want to start playing an initiator to give my team an advantage during engagements. What are good heroes to start with. I have some experience with Centaur although it hasn't been too successful.

Try two birds with one stone - there are several supports who can initiate "cheaply" (i.e. with no Blink Dagger) as well as those who can.

E.g. Warlock, Treant Protector, Jakiro (to a point), as well as some slightly more expensive ones like Sand King, Tidehunter, Earthshaker etc.

Clockwerk is an awesome initiator that can break those stupid stalemates, just be sure to have backup.
If your backup isnt too quick in a specific game then either pick up a force staff to escape them or a BKB to ignore magic damage and cause even more chaos.
Of if there is a rightclicker like ursa who isnt getting a quick BKB then you can consider blademail.

If you want a blink hero that can pick off heroes that are out of position then sand king is great, you need to have a blink but you dont need to wait for a long scepterless hookshot cooldown.

Warlock or Tide.

Though it's usually better to stay out of the "5v5 mid" altogether and farm an empty lane top/bot with a TP scroll in case they decide to push.

At the beginner level, you ideally want something that can affect 3-5 Heroes at once. Since you also want to Support, you also want something that can Harrass (so Ranged) and doesn't need much Farm, i.e. no Blink Dagger, prefferably something that can get Kills in Lane too.

That only leaves you with Venomancer imo. 8{(

Warlock fits the bill too as a DUAL lane support where you can get some levels - you dont need any extra equipment to get Chaotic Offering/ Fatal Bonds off. You don't HAVE to play him Aghanim's Scepter/ Refresher Orb :)

Tidehunter..

Great offlaner with Kraken Shell/ Anchor Smash..

As for initiation, get Blink Dagger, blink into enemy team, press R, initiation accomplished!!!

For bonus points late game, grab a Refresher Orb (but only after you have finished your Mekansm, Arcane Boots, Blink Dagger)

Thanks for the suggestions. I will probably try the heroes who can support AND initiate because they will probably help my team the most. However, if someone else picks a support then both Tide and Clockwerk look very interesting and tanky.

Initiators are divided into a couple categories: Supports, Utility and Carries that Own.

Support initiators include, as others have said, Warlock and Venomancer.

Utility are usually offlaners/junglers - Enigma, Clockwerk, Centaur. They often need a Blink Dagger.

Carries that Own usually need a BKB and a blink, but that's it. Perfect examples are Ursa and Phantom Assassin, but others who have different item requirements are Faceless Void and Spectre. These guys punish bad positioning to the extreme. Sure, there are better initiations, but there is nothing more fun than going balls-deep 1v5 as PA and seeing the other team have no idea what to do about it.

Most supports are more pickoff oriented becouse them have stuns, but most arent that good at starting a whole teamfight.
Sand king is probably the best support at starting a full scale fight.

If you want to use more obvious brute force then undying might be fun. He is more about walking in, dropping tombstone and daring them to fight you while you spam decay. He is a bit reliant on being in front though, luckily he is a really strong laner. If you manage to get a fast mekanism then brute fore their towers. So dont be afraid to take early kills for yourself.