I suddenly have to referee a soccer match.

Our company is holding a sports day, and because a junior colleague who was originally supposed to referee the soccer match suddenly had something come up and had to drop out, I ended up having to do it.
It's not that I lack athletic ability, and while I'm not a die-hard soccer fan, I watch matches occasionally, so I know the basic rules.
However, I am curious about how much responsibility a referee actually has during a company sports day soccer match.
1. How strictly should I watch for offsides?
2. To what extent should physical contact be permitted?
3. Is it possible to accurately judge touch-outs or goal-line outs without linesmen?
The match duration is 25 minutes per half with a 10-minute break in between.
Both teams are groups that play futsal frequently, so the skill level won't be bottom-tier, but I'm worried since I'm the one refereeing. One team is from my own site, and the other team is from the site right next to where I work.
Please let me know if there is anything else I should prepare.