a few points that are likely non-obvious to a so-say beginner, some already made upthread.
saddle height - worth getting this right as it makes a huge difference to the hard work required and comfort. A good rule of thumb is straight leg with pedal in line with the frame down tube and heel on pedal. This will thus be about right with slightly bent leg and toe on pedal. There's a formula based on inside leg you can use too. Either way saddle may end up higher than you'd have thought - you won't / shouldn't be able to get feet flat on floor if saddle is right height.
The ball of your foot should be on the pedal rather than the instep.
Saddle - paradoxically a soft or padded saddle is likely uncomfortable and wrong. A harder unpadded saddle is likely better - but the specific model and shape needs to suit you. I like brooks leather saddles but I have a different arse from yours so Brooks may or may not be right for you. Padded cycle shorts are not the answer to a bad (for you) saddle but are more about stopping chafing and long distance comfort.
If carrying luggage, panniers are vastly better than a rucksack.
Mudguards are a very good thing.
Helmet is debatable - wear one by all means, but suprisingly they don't appear to reduce injury on average when made compulsory like in Australia, so it's not clear cut more safety at all and may help or hinder in any particular case. Don't want to debate it here - but it's fair to say it-s not as clear cut as you'd reasonably think
Make sure you understand the gears properly. Not being patronising as it's not actually obvious unless told that it's somewhat undesirable to "cross chain" with biggest (high gear) front cog to biggest (low gear) back cog. It's also usually considered better to spin relatively fast in a lower gear than grunt in a high gear. At least 60 rpm to maybe 90rpm+ say. Typically use the biggest front cog (high) with the high-half of the back cogs for downhill, flat, and with a tailwind. Then move to the middle front cog for the middling back cogs and mild hills, and small front ring for proper hills with the big back cogs. . Low gears are a very good thing - so if you're struggling up hills it may be possible to lower the lowest gear - typically by putting bigger cogs on the back. May or may not be necessary - and may or may not be viable cost wise too. If it's an old mountain bike likely quite low already to be fair
Don't dismiss out of hand things like drop handlebars or clip in pedals as only for "experts" or "racers". Both make things easier. Maybe a future choice - and it is a preference not a " do this"
Tread isn't necessary nor at all desirable for road use. For various physics reasons smooth is best for push bikes on tarmac wet or dry .
Brakes - nearly all serious braking is front only. When braking hard the back wheel is almost off the ground and therefore can't give you any stoppyness (technical term). As a kid you, like me, were probably told to worry about going over the handlebars - whilst there is something in this ( just about) you do need to teach yourself to use the front brakes properly with confidence. Makes sure they're adjusted probably, cables free etc and given it's been laid up worth giving rims / blocks a clean with meths.
Buy a track pump - a so-say luxury that is basically essential. and pump the tyres fairly hard - towards the high end of whatever numbers are written on the tyres especially if you're a big lass. I'm guessing 60psi or more but read the numbers. My narrow road oriented tyres are more like 100psi for comparison.
Get a spare tube or two and know how to mend a puncture. Old tubes could be semi-perished if bike has been shed-bound for years - ditto tyres - check if they are cracked.
And a key hint for road cycling - stay out of the gutter - maybe 1metre from curb and take the lane properly if there's a bend, pinch point or especially if your are turning left. If you get beeped, f*&^k 'em . You will be safer and get fewer close passes
Anyhow -sorry for a "do this", "do that" tone - but typing on phone so harder to be poetic.
And welcme to the joy of cycling
BTW