the early levels of the game (levels 1-5 for most games) are dominated primarily by your lane staying power. you shouldn't try to get kills. you just want to scare off the enemy DG so you can have lane farming privileges. get the lane flag, kill the creeps, then back off and regen. 

if you suffer less damage than your opponent and regen more health/mana thany our opponent you'll dominate the lane and level up faster than your opponents. 

so get some regen items, get potions, get speed boots so you can change lanes faster and escape more easily, get teleport scrolls so you can retreat to base and get back to your lane faster. 

Don't listen to anyone who says flags are better then farming for exp. Capping a flag gives the same exp as 3 minataurs, so a single wave is worth like 2 flag caps. :/

Secondly, if your TB or Reg, just harass and don't fight enemy DG. This means with Reg try to get pot shots off at the enemy (get one or 2 shots off and run). With TB just spam them with fireballs (or ice mode normals) and run if they try to chase you.

Also, you can be quite a distance away from the creeps and still get exp. If you really have to, just stay a bit behind the tower. The enemy DG will kill all your creeps, then they come to your tower and you get exp as the tower kills them.