Here are a list of things new Warlocks often do, that you should avoid.
Remember that two simultaneous P-gestures, at any time, will make you surrender. Bear in mind that this also applies if one of the P-gestures wasn't your chosen gesture - if your opponent is paralysing one of your hands into making a P-gesture, and you choose to have your other hand make a P-gesture, then you will surrender.
Most often, however, the newbie mistake is to make two P-gestures without realising. Don't do this.
Mistargeting Counter Spells
This is especially common - you cast Counter Spell at the target of the spell you want to counter, not at the caster of the spell (except where the caster is also the target).
Realise that when you cast Counter Spell, you will counter all spells cast at that target - there's no point in trying to summon a monster when you're countering the opponent's damage spell on yourself in the same turn. There's no point in trying to confuse your opponent when you're countering their defensive spell in the same turn. Your own Counter Spell can ruin your own spells.
When you summon a monster, cast the summon spell at yourself. No matter how much this is stressed, people get it wrong. (Note - advanced players will occasionally cast a summon monster spell on a monster they control, which also works, but this is rare.)
Casting Two Enchantments
When you cast two control enchantments at the same victim, they cancel out. Make sure you don't cast two such enchantments at your opponent at the same time. (Control enchantments being Confusion, Paralysis etc.)
Targetting and Choosing Spells Too Early
You only have to choose and target a spell on the turn you complete the spell. Other spells can be cast along the way.
PSDF, for example, will cast Shield (P) on turn 1 (which you can target where you want), Magic Missile (SD) on turn 3 (which you can target where you want) and Charm Person (PSDF) on turn 4 (which you can again target where you want). You only choose the target for the spell you finish. Likewise you only choose the spell when you finish it. (You don't have to choose Charm Person on turn one, only on turn four - which means you can change your mind after turn 3 and cast Charm Monster (PSDD) instead.)
Not Watching Your Opponent(s)
Newbies often blithely go about casting their spells without paying attention to what the opponent's doing. There's no point in continuing to cast Finger Of Death when you're about to be hit with Anti-Spell, and can't counter it. There's no point casting Fireball at someone when they're going to counter it. Make the best use you can of your gestures, be ready to drop what you had planned and do something else. You have to be flexible with your plans.
To the Spells Quick Reference