Basic tips for all new players:
- If you're playing a support, support your team over helping yourself (get wards, get sentries, get dust, get smoke, let your carry farm, etc etc). Need to learn to play with minimal items and still be effective.
- If you can't stop something from happening, don't try. What I'm really getting at is defending towers, but this applies to saving allies or Roshan as well. If you see 3 people pushing your tower and you're alone...don't stick around trying to make something happen if it's a super low-percentage play. You dying + the tower falling is the likely outcome vs. just the tower falling. Needlessly dying accelerates the other team's snowball. No need to throw a cute little nuke and plan to run away if it isn't going to stop the tower from falling or killing a high priority target. Just let the tower fall and move on.
- Try and check the map as often as you can. This took awhile to get used to, but you need to glance at the mini-map and know where the enemy is as often as possible. During the laning phase especially, noticing that people are missing yourself should be your goal vs. depending on others to tell you people are missing. If you're lane is pushed out far and people aren't on the map...play with caution.
- Invisibility heroes during the laning phase should be countered early and often. If you're laning against a Bounty Hunter or Broodmother or Riki...etc etc...get sentries and place them strategically down the lane. They go from terrifying to manageable in these scenarios, and often you can kill them because they aren't used to having people detect them and get over-confident. Prioritize this over some other items if you can.
- Do not auto-attack creep waves pretty much ever (unless you're pushing). Generally, you want your creep wave as close to your own towers as possible. You attacking the creeps needlessly just pushes the lane further away, making your more exposed to enemy ganks. Keep the wave close, and if you take out the people in your lane, you can perhaps decide it's a good time to push.
- CARRY A TP SCROLL! ALWAYS! You should be blowing anywhere from 1000-2000g per game on TP scrolls. In the laning phase, a TP scroll could be the difference between you living or dying. A TP scroll could be the difference between your teammate getting ganked or surviving...or even counter-ganking the other team. If you don't have a TP scroll on you, you shouldn't feel good. Fun tip: If you have double-click casting enabled for items, double tapping your TP scroll will target your fountain and TP you there. Very fast escapes this way.
- Items: General rule of thumb is that if you're winning, save for game changing items (Heart, MKB, Abyssal...etc), if you're losing, spend your money on stat items (bracers, drums, ghost scepter, etc) to try and equalize the playing field. Lower your bar for target items (instead of a top tier item, target something more mid tier like a SnY, Drums, Halberd, Blade Mail). If the other team is starting to snowball, these stats or mid-tier items could be the equalizer you need to tip the scales in your favor before it's too late.
- Know the difference between long lane and safe lane. If you have your team's jungle next to the tower in your lane, you're in the safe lane. What this means is that the creep waves initially collides right near your tower, giving you a nice advantage early. On top of this, the enemy team is more easily ganked because you can have teammates pop out of your jungle and catch them from behind if they push up too far. If you're in the long lane, your goal is to A) not die and B) not die and C) sap as much last hits and gold as you can. If you need to miss entire creep waves of exp/gold to prevent pushing out too far, it's likely worth it than to die. You'll learn over time when you can take bigger risks in the long lane and which heroes do better there. You want your carry in the safe lane, and you want to send someone with them that can do a great job protecting them by either harassing the other team with ranged attacks/spammable abilities, or prevent/heal damage on the carry.
- Important timers: Runes spawn every 2 minutes on the 2 minute marker, regardless of when the last rune was picked up. A new rune won't spawn unless the previous was picked up. Roshan spawns 10 minutes after he dies. If you attack a camp of creeps and run away from them at the X:53 mark on the clock, that creep camp will get an additional group of creeps spawned there on top of the previous group you just pulled. This is great when you're farming up for extra gold or attempting a pull. (I won't get into pulling, but if you've got all this other stuff largely down, you should read up on pulling).
- Learn as much about enemy heroes as you possible can. While you're waiting for games to queue, check out the library tab and read up on abilities other heroes you're not so familiar with have. Not knowing what the enemy can do would easily be the difference between you feeding the whole game and you dominating the other hero.
...that's probably plenty to chew on.