For quality instruments sold by a family run business thats been in the heart of Belfast for decades I recomend http://www.matchettsmusic.com/ . They have 3 shops in the Town, the piano store which is at the back of city hall heading towards the Dublin Rd, the classical store which sells sheet music and classical woodwind and string instruments, the rock shop is situated just down the street towards the inst end of the city hall.
http://www.marcusmusicbelfast.com/ is located at 125 Royal Avenue and sells more modern instruments such as guitars, drums etc.
http://www.belfastmusic.co.uk/ is an online store selling music supplies.
Editing:
If you are a mac user I recomend using garageband as it's an easy to use but effective tool.
If you are a PC user not to worry Audacity is an open source music editing software which is just as effective.