If you have not cycled on roads for many years – most important is to learn how to cycle safely. Start on quiet side roads with no or little traffic and get used to the gear changes. Many mountain bikes have 21 gears or more, when only 5 or 6 would be more than adequate. Find a method of changing between gears that is easy and gives the ratios you want. On my bike I have 21 gears (3 at the pedals and 7 on the rear wheel) – for simplicity, I sometimes use the 3 gears only, leaving the rear gear at a mid location. Once you gain more experience you can mess about with the full gear range.
Adjusting the saddle height might take a few goes to find the optimum. If it feels uncomfortable, take the bike to a local cycle shop for advice on a suitable replacement saddle.
I also check the weather before setting off on a bike ride, so that I am certain I won’t get a soaking.