You are a new player, so you should expect your stamina to be bad. If you full alt it will be harder to increase your stamina without specifically playing speed maps or maps with long streams that are outside of your BPM comfort zone. I full alt and I've only ever seen speed improvements from doing long stream practice. I also think it will take you longer to get used to maintaining high accuracy if you full alt since its less intuitive to tap to a rhythm with two fingers.
But if you still want to do it, I'll let you know the secret...
To full alt, you press one key, then you press the other. Then you do that again until there are no more notes.
There are no special tricks to it, start with easier maps if it's too hard on what you can currently do without alting.