Basically, you have to pay attention to the traditional score at the top (the stuffy "piano music" that everyone hates
)
A red bar is an incomplete one; that is, one that doesn't fulfil 4 crotchet beats in its bar. A crotchet is the last note in the first red bar.
In that bar you have 4 quavers - 2 quavers = 1 beat, so in total you have 3 beats in that bar. You need to get an extra beat somewhere, by making some notes longer or using a strategically placed "rest" note, or both.
In the second red bar, you have a minim, which is the hollow circle with a line coming off it. A minim is 2 beats - so you're missing another two and will need to pad the bar out as described above.
Unfortunately to get into GP tabbing you have to get into all this nonsense, I'll give you a quick rundown of the notes.
Semibreve - hollow circle - 4 beats
Minim - hollow circle with a line on it - 2 beats (equivalent to 2 crotchets)
Crotchet - Solid black note with a single black line - 1 beat
Quaver - A solid black note with a black line with a little "tassle" on the end - or two can be joined together with a horizontal line. -
Semiquaver - Same as the above but with 2 tassles, or can be joined with two horizontal lines.
Demi-semiquaver - same but 3, etc...
Thats the basics to get you started
My advise is to pay attention to the little number in the bottom left of the tabbing window, it will tell you much of your bar you've filled.
