Hi mcslullaby!
I'm also new! Well what I know just briefly is that obviuosly at first Nero was all for Agr. calling her "the best of mothers". However, she began to lose her influence over him when he began to rebel by having relationships with Acte and Sabina and thought that Agr. was too much of a threat and did everything in spite of her.
Claudius saw his marriage to Agr. as strenghtening the Julio- Claudians ties and uniting the two familes. I think that according to Tacitus, later Claudius became aware of her power and ruthlessness and so was contemplating removing her and Nero as heirs!
I hope that helps, if you need any more detailed info I'll try to help!
