>>536197663
It's been a long time so I won't answer all of the questions in the first part. IIRC, BG1 has a pretty organic progression, though it does reward thorough exploration. Your early level mages are going to suck and die in 1-2 hits. Use your sling, your Sleep, your Grease, your Web, whatever small advantages you can get. You don't have cantrips and you're squishy as hell. Grab the companions that seem fun. Those you don't recruit won't magically level so these are choices you commit to for better or worse.

>Should I go Mage/Thief, or Mage/Fighter
This last part is pretty funny. BG1&2 are old games based on AD&D 2E. It is technically possible to achieve what you're describing, but because of the way dual-classing works there - you first need to level your (martial) class to X, and only then can you switch to your (caster) class - I would definitely not recommend it to a first-time player who's just trying to have a fun experience instead of following a spreadsheet of when exactly to switch classes for the optimal gish. Dual-classing will also gimp your early game experience, forcing you to play a Thief or a Fighter when you really want to be a Mage. This isn't like BG3 where you can pick whatever class you want every level and where respec is unlimited. Also you have to be human for dual-classing. Old games, old edition, old rules. It CAN be done but keep that in mind and google a bit if you really want a hybrid character (human). There's also multi-classing for non-humans. This comes with pros and cons compared to dual-classing, depending on your archetype - again google is your friend here. It's less restrictive (because you're not forced to be human or spend BG1 being a Thief) but also progresses slower. Character limit forces me to cut this short, but it's something to plan now if you want to go all the way to ToB with its epic levels. My recommendation: Martial or Fighter/Mage (multiclass). Oh, and google race restrictions if you want romance Viconia.