A typical beginners question

hi, due to the (i hope) simplicity of this question, any and all sarcastic comments are accepted and forgiven! i would also like to apologise for posting such a basic question on this forum, but iv got to start somewhere!

i have a scroll bar with justified text, dynamic or course. but when i highlight a particualr line to format, eg, make bold, change colour etc, not only does the highlighted text change, but so does the rest of the text.

where am i going wrong? i cant seem to figure this out!!

can anyone point me in the right direction? (cue sarcastic replies!!)

thanks all. much appreciated.