Do I always have to start a solo on the root note of the chord or can I use any note in the chord? Example the chord is C# 7. Can I start the solo on a B note? Thanks
Try listening to existing solos, it's a good start point. You'll notice that they can and do start on any note, although the root is fairly common.
There's also the factor of which beat do you start on. Again, it could be and is any beat, or even in between beats. Although the first beat in the bar is fairly common.
There are no hard and fast rules - in fact, no rules relating to this. However, when first learning to solo, it's reassuring to start on the root note on the first beat of the bar. That concept hopefully won't last long, and you'll find you can put in an anacrucis which most likely will not start with the root note. move on to trying other chord tones, then you'll realise that actually, any note from the key - then any note at all (!) will work. Long, sustained notes, though, sound better when they're chord tones.
You can start a solo on any darned note in the scale of the chord.(if we're playing inside,thelonius monk said[and Eric dolphy would agree]that you can use Any of the 12notes if you can figure out how to resolve it correctly,but I digress)..c#7 is a chord with a major 3rd,so we have a six note scale(no "fa" , or major 4th , if you will, over a major7,nor 7 chord,unless you a lazy psychedelic blues trained guitarist such as myself, if you use fa you can't use the 3rd of the chord 'mi'during fa and you have turned it into a sus4 chord during 'fa time',more digression...) So: the normal notes to use over c#7: c# d# e#(f) g# a# b That's it.