You have to balance your throttle, elevator and speed to maintain altitude. You can max out the throttle, but work up to that slowly, don't do it in the first few attempts.

From a hover, the more throttle you give it, the more forward elevator you'll need to keep it from climbing and the faster it will accelerate forward. As you pass that threshold in speed I mentioned above you'll experience "transitional lift" and the heli will want to pitch up and climb, so watch out for that. It's a balancing act that requires coordinating a number of controls in order to keep the heli going in the direction you want it to.

Instead of just flying straight ahead down the lawn, you might want to try slowly working from a tail-in hover to gentle side-to-side movements, but keep the heli pointing the same direction. Slowly work up to wider and wider movements as you become more comfortable. As your side to side movements start feeling natural, start turning the noise a little bit so that it's pointed more towards the direction the heli is moving. Then slowly increase the amount you turn the heli as you gain confidence until eventually you are doing 180 degree turns at each side. Then you can start working your side to side movement into a continuous figure 8 which will really give you a workout coordinating all the controls.

Lot of people find this site helpful for learning to fly heli's:
http://www.dream-models.com/eco/flying-index.html