Here are some general hints (these are no joke despite sounding either obvious or simple)
1. Don't Die... EVER: If you are dead, you are not fighting, gaining expo, the other team can push and they got money for killing you
2. Don't go for kills, play it safe: Untill you have a really good idea of the game, going for kills will just get you killed, stay alive and harass the enemies, only kill if it is a sure kill. The point of matches (in general) is not to kill the enemy demigods, it is to kill their citadel or control flags, or defend forts, killing is just a way of keeping your enemy away for a while and getting some money, don't focus on it.
3. Buy HP/Armor Items: I cannot stress how good these are, especailly for weaker Demigods
4. Stay In the Fight, but Don't Take Damage: Just by being near the enemy creeps dieing, you gain exp, thats good 
5. Ramp Your Items: Don't Save the the UBER ITEM off the bat, buy cheaper items and you can always sell them for better ones.
6. Teamwork: 2v1 makes odds great for you
7. Know the Format: Some formats only require you to control flags or defend fortresses, don't go off doing unimportant things
Lastly, I would recommend trying the game against AI's untill you are decent if you don't wana get stomped. I played a tournament on normal, hard, and nightmare plus around 10 lan games vs computers (and a ton of just skirmishes) before I even stepped into multiplayer and I STILL got stomped when I started. Just live and learn my friend.