How long should one work out a day?

Heyo folks, I went to the gym today for the first time in years. I Don't know how to use half the machines and I'm not sure what weight I should lift or how many reps etc etc

I spoke to a guy who told me what I should do for biceps so I went and did that until I had difficulties continuing. was maybe 13kilos then 10 on the bar then 4kilo dumbbells. was there for twenty or twenty five minutes doing biceps before I went for a shower.

I guess what I'm asking is if I want bigger arms and to lose some stomach what are the main things I should be doing?

say I'm there for a hour...

should I just be doing say three sets of 13kilos with ten reps each.

then doing three sets of 5 kilo dumbbells.

repeat those for half a hour then do sit ups or something?