First bike ride in 27 years but need advice

[Lincov]

I agree with pp that one of the best things you could do is get your bike serviced, so it's working as it should and and set up right for you. In terms of gears, I only avoid the opposing pairs of gears, I use all the rear gears while in the middle ring at the front. A lot of ratios can be achieved in different ways, so it often depends what you're going to want to do next e.g. if there's a hill coming up you might want to be in the smallest ring at the front so you can easily access all the lowest gears.

Very well done for getting back on a bike (more than once!). I know how tough it is, but it's so rewarding.

 

[Rickshaw Phil]

Well done @Andy1974

As far as the gears and the saddle height go, @MikeW-71 has summed it up pretty well: Saddle height does need to be higher than you'd expect (but when you're a bit more confident with it) otherwise it tends to make pedalling hard work

If you stick in gear 2 at the front you can use all of the gears at the back so choose whichever feels okay for you. As already mentioned, it suggests the gears need a bit of an adjustment if it's thrown the chain off.

Keep those pedals turning.