i would just buy a sketchers pad and some decent pencils and start sketching, dont worry about them being rough and just keep going, then pick your favourites, make them neat and presentable and take those out and put them into another proper portfolio where they can be protected in wrappers. i wouldnt worry about colour at first, get your line work and concepts, composition etc down first and move on to colour and paint later. you gotta crawl before you can walk, walk before you can run etc. Im not a tattoo artist but i am an artist and this is what i would recommend ๐ good luck x