Hi Irene, to get to Algeria from Tunis you can either fly or get the louagge from Tunis, Bab bhare.
If you visit
www.tunisair.com the website is in French but you can pretty much find your way around and you'll be able to find the information for flights from tunisia to algeria.
Getting on the louagge may seem the cheaper option but its a lot of travelling. I dont know if it would be worth you visiting your local travel agency as they may be able to organsie all the flghts and hotels with an intertinery so to speak, as you could get flights and hotel in tunisia and then the same in algeria and back again, it would also save you having to book everything separetly especially if you've never been to Tunisia or Algeria before