Can I use my 1st generation iPad ans a second screen for my Mac Laptop
I think you will run into problems trying to do this with a 1st generation iPad, as most software I've come across to achieve this sort of thing requires iOS 7 and a 1st generation iPad can't officially run this version of iOS.

In general though, if you want to use an iPad or iPhone as an extra display for your computer you will need to use 3rd party software to do this, such as Duet Display. 

I have used this app to set an iPad up as an external monitor for an iMac and it works well for some workflows. How well it works for you will depend on what you're trying to do and the hardware you're using.

Duet Display does work with both iPads and iPhones and works with both Mac and Windows. However, I can not share any personal experience in using it with a 1st generation iPad as I think all versions of Duet Display require at least iOS 7.

There are also other options you can investigate, but I have no personal experience with them and don't know if they will run on a 1st generation iPad:


Air Display
iDisplay
Mirroring360


Hopefully this answers your question one way or another.