I'm not really qualified to give advice since I just recently transitioned from League but I'll try anyway.
Before my transition I was a high platinum support player, now playing DotA I noticed many similarities between the roles and I'm having great success implementing that knowledge in a match.
So without further ado here are some tips I always have in mind:
1. Pick a hero and stick with him: learn all the ins and outs, read a few guides and watch a few professional matches from that hero's perspective, write down important things like what is he doing at any given time, item timings, smokes, ward positioning, etc. and then experiment with him; see what works in your particular skill bracket. I would recommend Phoenix or Lion.
2. Be mindful of your gold: sometimes being an only support means you'll be gold starved, so be smart with the little gold that you have. You don't have to always TP from your base to a lane (Unless it's an emergency), double stack pull or pull through and try to get as many last hits on the neutrals.
3. Provide vision for yourself and your team: buying wards is a must but most players waste them on wrong spots, for instance they would ward too defensive when they have an aggressive lineup or vice versa, think about a ward spot before you plant it; if they have a Natures Prophet ward his cliff, if they enemy support is de-warding you, be creative.
I've noticed it's best to spend your first two wards: one in mid to secure your mid player with an advantage especially if he's playing a melee core like Ember, Magnus, ... since it helps alot.
And the other one to get deeper vision in your safe lane, where your will be, for better zoning potential and to keep tabs on the enemy.
4. Rotate and smoke gank: if your carry can handle opponent 1v1 in lane you should leave him and help other lanes, always carry a TP, get one early and use it to counter gank it's so effective.
5. Create gold for yourself and your team: if you have someone like Sven try to stack your hard camp which is easy since you're mostly be pulling anyway on X:51 to deny your creep weave. (If you're not zoning the enemy offlaner you don't have to be in a lane)
6. Focus on improving and PMA: I don't really care about my teammates so I don't try to stop them arguing when the game dissolves in to chaos. I try my best to win, always!
Be liberal with the use of your mute button, and if you see someone is getting triggered use your emotes like: >Sorry, sometimes it helps them to calm down (By saying sorry you acknowledge that the misplay was your fault and now they have someone to blame, it can't possibly be their fault for going 1v5 ever, and you want them to blame you because they are fragile snowflakes, who can sell their items and start feeding at any given time.)
Don't flame, don't try to debate things with them most of them are too far gone.
Also, be smart about your plays in general. Try to always think your moves through, at the start of the game try to come up with a plan how is your team going to win this match, identify their strong heroes, weaknesses, itemize accordingly. If you want to improve you can't let yourself play a game on autopilot, always think about your actions.
Lastly, find a hero you like and have fun. I think Void is pretty strong and I can play him decently but he's not particularly fun to play (At least for me). So I tried Phoenix and I'm having a tons of fun, which echos my ability to perform better in a match:
If you have any questions feel free to ask.