A 4-ft. T-square for drywalling can come in handy for other projects. Besides being helpful in drywalling, it can be useful when laying out patterns or cutting on full sheets of plywood or particle board. The square is accurate, stable and will give you a full 48-in. straight line. Some newer versions come with a head that can be adjusted to various angles, then clamped in place.
Wallpapering can involve a steep learning curve and most professionals advise starting in large rooms. Save bathrooms for last, or at least until you’ve got your confidence.