Welcome to our Forum, Synapse!
You're smart to start with Guitar and the Guitar Star challenge. That's by far the most difficult to get. Also, be sure that your Sim doesn't learn any other skills until Guitar Star is complete because it will just make it even harder.
There's also one opportunity related to the Culinary Career that has been believed to be bugged. It's the Bad Apple Opportunity and you're asked to take a bad apple, plant it, and grow 10 more bad apples in a week. I'm not aware of anyone who has been able to complete this challenge without cheating.
What kind of problems did you have with the Omniplant series? I've completed that one several times. Perhaps I can help.