Maximise pit entry speed, but watch out for the lines both going in and going out or you'll get a penalty.
You don't have to put any in, but once it has started you can hit "X" to stop fuelling at any point. Keep an eye on your fuel consumption and plan to have sufficient fuel to cover the laps until your next stop (or the end of the race) at the rate you intend to consume it.
Every car has a 100-litre tank.
That depends on the race requirements. If a certain tyre type is mandatory, you must use that tyre type on at least one lap during the race or you'll get a penalty of a minute. If multiple types of tyre are mandatory, you must use all of those tyre types on at least one lap during the race or you'll get a penalty of a minute.
Otherwise you are free to choose what tyre types you wish to use and in whichever order you choose.
For reference, it doesn't count as a lap if you pit on the final lap with the chequered flag waving, nor if you pit immediately after the race starts - which you can only do if you're very far down the grid. However you can pit at the end of the first lap or for the start of the final lap and this will count as one lap of using the tyres.
If you're talking about this week's Daily Race C at Brands Hatch, no you don't. The Hard tyre is mandatory, so you only have to use that one. The Medium tyre is also available but not mandatory.
You can therefore run any strategy you want - even no-stopping - so long as you use Hards on at least one lap.