At character creation you can raise strength higher than 18, to 18/38, 18/58, 18/78, 18/98 or even 18/00 or so. I don't recommend doing so.
I don't recommend starting off with "somewhere between 9 and 14 Constitution". The reason is that you spend attribute points without getting a noteworthy benefit. The hitpoint adjustment for con is the same with 9 and 14, +0 per level. For 15 con you get +1 HP/level, +2 HP/level for 16, ..., +5 HP/level for 19.
I recommend starting with 14 con so you get the 10 extra HP at the beginning and if you feel weak you can raise it to gain an immediate benefit, retroactively +1 HP/level for each level up point you spend for it. So a level 15 character who has 80 HP at 9-14 con will have 95 HP at 15 and even 155 HP at 19 con.
But if you start with 11 con the first 3 stat raises at level up won't add any HP, it will take too long to gain any effects, so you might as well start with 9 and keep it down.
For strength it's similar, you won't get any combat bonuses with 9-15 str, so either keep it low at 9-10 or raise it properly.
You can't choose your class, you start as fighter. Maybe you'll find an opportunity to switch to another class, be patient. Do what you like the most, your character can't do more than one class at the same time. Fighters can fight the best, thieves sneak, backstab and steal, mages can cast spells.
For effective combat it's important that you have weapon proficiency points in the weapon class you're using, the more the better. You start with an unused proficiency point, maybe you'll find a way to use it.
Don't feel rushed to experience things immediately, be patient, let the game unfold towards you, you might get quests long before you can solve them and meet potential party members long before they might join you for example.
You can initiate dialogue with your party members, it can be very useful sometimes.
