First off - yeah, that's pretty much how we all started. Go to!
Don't be afraid to make something up; also don't be afraid to say 'Hey, y'know, I think we messed that up last session, and I'm not going to repeat it. Here's what we ruled 'on the fly' last session: *recap*, and here's how I'm going to rule it from this session on: *explanation*'. For the most part, its better to rule in directions that
limit rather than
enable: players are much less likely to complain at being given more power than given less. Its also plausible to say 'okay, I need half an hour to prepare: why don't you guys order a pizza or something' in the middle of a session if the players go an unexpected direction.
Feel free to ask for assistance on the boards - there's probably no question which SOME DM on the board hasn't had to give some thought to!
My only concern is that I think my players are scared of magic users. It would be super if one would play a wizard or a sorcerer but I think the responsibilities of learning spells and their effects worries them. Is there an easy way to ease the tension that magic users create? Should I make a NPC that can introduce magic into my campaign? Should I pull aside the cleric or bard since they have some experience (only a few sessions but still...)? What do you think?
Very common concern for new players - and understandably; it really is tougher to play a spellcaster: it feels like there are more rules and things to know.
I'd recommend,
don't force them to play a class that they're not comfortable with - you've got a fine mix there. It gives you an opportunity to have that 'sorcery is a little evil' feel to your world, and that's fine: it leaves a lot of mystery in those magical encounters.
You-the-DM may need to watch out, as you create challenges for them: A high-level wizard who gets the drop on the party may well be a match for them. Also, monsters which can only be hit by magical means will be dramatically more challenging, at least until they start having magical weapons, etc. Read the monster descriptions, don't just rely on 'CR'! Some problems may take more creativity from your players (it usually does, anytime you're missing ANY of the core classes), but that's not a bad thing at all.
If there is a distinct definite need for the party to have a spell-casting wizard, you might let them encounter an NPC who can fill the void and might provide a plausible 'trainer' for somebody who wants to multi-class into some wizard levels. I'd recommend that this NPC be *lower* in level than the average level of the party, and be very clear that they are not going to stick around indefinitely. It's altogether too easy to wind up with a 'Here's a challenge which only the NPC can overcome; the players get to watch and wish they'd played a wizard,' and that can get awful boring awful quickly.
Best of luck, and let us know how it goes!