Welcome to the dark side!
First thing you should have in mind as a new GM is to take it slow. Don't go for the most advanced options in the beginning. The first scenario you run should be something pretty simple. Just a few story mode checks and then a simple combat encounter. Use only one type of enemies, normal monsters without any special actions (and no spellcasters). The first scenario could be as "railroad" (not sure you're familiar with the term, but it essentially means that the GM has a set idea on how the story will go, with little to none opportunities for the players to change the major plot) as you want. If you're playing with others who are experienced roleplayers, discuss this before so that you don't have to deal with players wanting to head off in directions you haven't planned for. The players should be ok with this to let you as GM learn the system.
Then, after a session or two you can start to add the more complicated stuff. List of advice, in no particular order:
- Make preparations for different choices made by the players. You can't cover everything they may come up with, but if you have 3 different prepared paths for each major decision (and ways to get sidetracks joined with the main story) you'll probably be fine.
- Before each session, sort out what creatures, actions, event tracks, and locations you need. Also try to set aside stuff for unexpected decisions by the players.
- Make enemies stand out, by givving them different characteristics and special actions.
- Use the ACE pool as you see fit. Not using it at all means the players will have a quite easy time. But you may also elect not to use it all if you misjudged the challenge posed to the players or if they are unlucky and you feel they deserve a break.
- Try and make up a list of stuff that can happen when the players or NPCs roll boons/banes, comets/chaos stars. It will come in handy during play if you feel uncreative when the roll is resolved.
- Add fortune to the party fortune pool often. Whenever you feel the players (or you) did something fun, memorable or well roleplayed add one fortune. It doesn't matter if the actions was a failure or success or whether it was stupid or smart.
- Try and make combat encounters a bit more exciting. Work with multiple types of enemies, add nice location effects (doesn't have to be on a card), use waves of monsters combined with Rally phases. Try and make combats last a longer than a few turns by using ACE defensively or by having NPC use defensive actions (Guarded position and active defenses).
- Use the event track to play track success of players in complicated story mode situations. Perhaps they need to find a beggar in the slum before the main villain, then have an event track that moves ahead a step if they succeed on a skill check and track the villain's progress on the same track.
- If you feel you have too much things to track, quit tracking recharge for your NPCs, just let them use actions as you see fit. This requires players who trust you to do the right thing though.
- Use insanities and diseases to get combat focused PCs worried.
- Challenge the PCs in areas of expertise where they don't shine. Get the players to see the fun in failure and how it drives the story
- And well, most importantly, make sure to have fun together!
After another 5-10 sessions you may want to start using even more advanced concepts:
- Let the players move the story. Don't say no, just let them try and see where the story goes. This requires some ability to make up stuff on the spot, but is really fun if you can pull it off. The WFRP3e system is very well suited for this in my opinion.
- Make advanced encounters, for example social encounters where the players can use social actions in encounter mode. Make encounters that are a mixture of event tracking and combat.
- Make your own actions, locations for your NPCs.
- Start using house rules for things that you think need them.