1. Complete in-game tasks and Contracts
This is what I find to be the best method of leveling up quickly and the best way to earn Soul Gems hence why I placed it in first place. There are a huge range of different types of games availible for many different platforms, even including Andriod and IOS. The task that you have to complete in the game is displayed under the rewards that you will recieve for completing it. From my experience all browser games have easy to complete tasks and had me leveling up through the first levels each time I completed one.
2. Posting on threads/leaving comments on games
When I first joined Gamehag I posted comments on threads to level myself up. For every comment I posted I got 10% of my level worth of experience. I would certainly recommend doing this throughout your first one or two levels but refrain from spamming as it's annoying for other users and potentially lead to your account getting banned.
3. Creating threads
Instead of posting on threads why not create one? Their are several sections of the forums that I contribute towards, it's a fun way to contribute and I earn XP for doing so! Of course like mentioned above, refrain from spamming useless posts in the wrong sections as it's very annoying and could lead to your account being suspended.
This is it for all my methods so far. I hope this helped in some way or another! If I find anymore methods I will not hesistate to post them in the comments section.