triplets and 1st ands second time bars

Triplets: see https://musescore.org/en/handbook/4/tuplets
1st and 2nd time bars (aka voltas): see https://musescore.org/en/handbook/4/voltas

If you need more help, come back and explain in more detail what you are trying to do, how you are trying to do it, what you expect to happen and what is happening instead.

As a new user it is always useful to try to get familiar with the handbook. Most things are explained there but it can be a bit of a trial to find it. Here is a good place to start looking https://musescore.org/en/handbook/4/full-table-contents

I recommend this to every new (or "new-ish" :-) user I communicate with. Similar to what Steve suggested, take an hour or two and simply read the entire Handbook (https://musescore.org/en/handbook/4). Don't try to memorize it! Just skim through it and familiarize yourself with the contents! But read it all!!! Then you'll have an idea where to find things when issues come up in the future. "Didn't I see something about that in the Handbook?"

Welcome aboard and best of luck!!!

I experienced this when I first used MuseScore, as well. And I hope this additional resource helps.

MuseScore 4 How to Add Triplets and Tuplets
https://www.youtube.com/watch?v=j8Ni8n8_TNA&t=13s

Just remember to select the note dynamic that you want in duration, then hold down the Control key + the number (i.e. Ctrl + 3 or Ctrl + enter the number of notes for that duration needed).