2003 Vauxhall Astra flicking out of first gear

If the Astra has high milage a replacement gearbox should be sought.

The car has done 90k and the flicking out of 1st gear happens if I rev over 2500 rpm, so I'm not sure if a new gearbox is needed, but also the gearbox oil is leaking a bit under car not sure if it has anything to do with it and if so how do I check the gearbox oil level?

To check the oil level, you need to raise the car so it's high enough to allow access to the oil filler plug on the side of the gearbox, making sure the car is level after being raised.
The filler plug usually has a square head or is recessed for an hexagon key. The plughole is machined to be in the correct position when the car is level.
Without axle stands this type of diy becomes very dangerous, and I would strongly urge you to either buy or borrow, for the sake of your own safety.Also this will not cure the jumping out of gear problem. Good luck.

its not a gearbox that is required all you need is the selectoer box located on the top of the box available from the dealers around 90 quid easy to replace if you have the right tools for the job a mechanically minded hope this helps