Articleship query

hello , i cleared my ipcc group 1 for november 2015 ,my queries are :

1)  im confused whether i should join articleship now or after writing group2

.2) after joining articleship if im not satisfied ,can i change the firm? is the process for change is difficult?

3) what should i look after,while selecting the firm?

4) its my 5 th attempt , can i get in Big 4?

thank you