First thing you should do is identify what deck archetype you are playing against.

Assuming this is a turn 1 [[Mana Wyrm]], you are most likely playing against Tempo Mage.

Second thing you should do is be able to recall common decklists and be able to come up with a plan to deal with the strongest drops at each mana.

Tempo Mage 2 mana common plays include [[Sorcerer's Apprentice]] [[Mad Scientist]] [[Frostbolt]] [[Flamecannon]] [[Unstable Portal]]

Tempo Mage 3 mana common plays include [[Arcane Intellect]] [[Flamewaker]] or a secret of some kind.

You should pretty much be able to disregard Unstable Portal, Arcane Intellect, a secret, and Flamewaker in this situation because they either do not impact the board (unless something ridiculous comes out from unstable) or you can most likely deal with whatever they played directly.

Third thing you should do is recall the win condition of your deck

Assuming you are playing Aggro Shaman, what kind of cards are in your hand? Do you have a lot of spells? Or do you have a lot of minions? With the hand you have, are you going to try to burn the mage with spells to the face or develop a board while fishing for spells?

Aggro Shaman 1 mana: [[Earth Shock]] [[Lightning Bolt]] [[Rockbiter Weapon]] [[Abusive Sergeant]] [[Leper Gnome]] [[Sir Finley Mrrgglton]] [[Tunnel Trogg]]

Aggro Shaman 2 mana: [[Ancestral Knowledge]] [[Crackle]] [[Lava Shock]] [[Flame Juggler]] [[Totem Golem]] [[Knife Juggler]]

You wouldn't want to play ancestral knowledge since it's too slow, and you dont want to play crackle and lava shock except on face in this matchup.

Knowing this, plan your turns.

For the most part, you should play a 1 mana minion if you have one. If you have 2 2-drops, you can play one of them not named totem golem. Look at your cards and come up with a solution for Flamewaker (Crackle, Earth Shock). Often times this matchup becomes a race for face damage and lightning bolt is way too good to spend on a mana wyrm if you can kill it with a minion of some kind.