In Dota exist a few efficient methods to earn Gold:
Every round 5 basic gold from round 5
Win streaks and Lose streaks:

• -For a win you always get one bonus gold for the win and the first streak starts at 3 wins with an additional gold, then 5 wins 2 gold, 7 with 3. (Get's reset after more than 8 wins)
  
• -after a loss the bonus is 1 gold for 3 losses and 2 gold for 5

Most important income source not depending on the outcome of rounds:
Interest
Interest is earned on every 10 gold in the Bank (at the beginning of the round) up to 50 gold.
10 gold = 1 interest
20 gold = 2 interest
...
>50 gold = 5 interest
Reach 50 gold as fast as possible and then start spending more freely like some XP every round and
always spend all gold down to 50 if possible.

Most important time to build an economy!
To get streaks plus some interest already either:

-get a winstreak if you were lucky with the rolls (most money)
-keep a losestreak if you were't lucky (faster money & this early damage doesn't really bothers later)

You can tank through the early game phase and recover if you build up a bank behind! Worst case would be a win lose win lose scenario, which gives the least money and the most damage.

Don't spend any money on rerolls or XP yet.

Depending on how the early game went you need to decide:
-over 65 HP and over 50 gold play slow and build a good team or buy XP by spending gold down to 50! every round
-under 35 HP spend ALL your gold. Most important try reach at least level 8

Always upgrade higher tier units first to stay in the game longer.

To have a real chance it's important to be over level 8 by round 33.
First try to have ALL units on level two or higher before buying level 10 if not spend gold on rerolls until its the case.
Spend your gold explosively: Decide in a round and reroll/buy everything which helps and hope it's enough because no more interest.

General advice:
- Rather choose one allience to max (i.e. 6 Savage units) than multiple alliences (i.e. 2 trolls 3 knights)
-spent money in the round if not directly impacting the game to earn more interest (since its couned at the beginning) so even if you spend down to 39 gold but had 50 at the start you get 5 gold interest.
-if you bank is full but some unit appears two times in the buy menu but you cant upgrade since the bank is too full for the second one:

• just put one more unit than allowed onto the battlefield
  
• upgrade the uni
  
• remove unit from battlefield before start of round otherwise some random unit will be removed
  
• be quick

Don't hesitate to sell tier 1 units even if level two and on the battlefield always use Alliences!

Recommended Alliences for beginners:

• Early game:
  Savage 3x
  Warrior 3x
  Assasins 3x
  
• Mid game:
  Druids 2x
  Knights 4x
  Trolls 2x
  Demons 1x
  Dragons 2x
  
• Late Game:
  Savage 6x
  Druid 4x
  Assasins 6x
  Dragons 2x

Try to get:

• -no loss in total
  
• -50 gold in round 12
  
• -with my favourite allience (Wild/Druids) have the "Completing the Cycle","Summoning Stone","Tooth and Claw" and the "Mekansm" for the Lone Druid (maybe also "Refresher Orb" if possible)
  
• -after having full Wild and Druid Allience add Terrorblade for Demon Hunter and Demon allience (otherwise demons might be a weakness) and also Kunkka for full Warrior allience/could also add more Lone Druids if lvl 2 or higher
  
  
• Example: