Craft Badges from trading cards.
Complete Steam Achievements for extra XP.
Buy and Sell Trading Cards on the market.
Participate in Steam Sales events for extra XP.
Set Profile Backgrounds and other customization.
Earn XP from Steam Quests during seasonal events.
Trade Cards with friends.
Activate Game Keys sometimes they give you XP.

Originally posted by terminator:

Craft Badges from trading cards.
Complete Steam Achievements for extra XP.
Buy and Sell Trading Cards on the market.
Participate in Steam Sales events for extra XP.
Set Profile Backgrounds and other customization.
Earn XP from Steam Quests during seasonal events.
Trade Cards with friends.
Activate Game Keys sometimes they give you XP.

How does earning Steam achievements give you XP?

Originally posted by Beardface31:

Originally posted by terminator:

Craft Badges from trading cards.
Complete Steam Achievements for extra XP.
Buy and Sell Trading Cards on the market.
Participate in Steam Sales events for extra XP.
Set Profile Backgrounds and other customization.
Earn XP from Steam Quests during seasonal events.
Trade Cards with friends.
Activate Game Keys sometimes they give you XP.

How does earning Steam achievements give you XP?

Earning Steam achievements increases your Steam level because they give you XP (experience points). This XP goes towards leveling up your profile, which is reflected in badges tied to game achievements. The more achievements you earn, the more XP you get, and the higher your level.

If you create badges during the winter or summer sale, you get a seasonal trading card for free. If you sell the seasonal trading card, you can get buy few more cards to complete more badges. Those badges give you free seasonal trading cards. Hopefully you see how this works and how it can self-perpetuate for a while.

If you choose not to sell the seasonal cards, you can use them to go up levels instead. This is faster but requires you to put some money in your Steam wallet. I personally find it wasteful to spend money on Steam levels and would rather spend my money on buying cheap game bundles.

The fastest way to level up is by crafting badges.

You can also:

• Earn the Community Badge by completing tasks.
  
• Redeem Steam Points for the Seasonal Badge.

Here's the FAQ for Steam Levels and XP.

Originally posted by Excel:

how to level up faster

The answer is always crafting badges.