Hi @MSrinivasan,
RE 1 & 2:
I suggest working through this tutorial. You can use either pre-trained classifiers or ones you train yourself. It's up to you. 
If you search the forum you'll find additional content:
- Search results for 'help train silva classifier' - QIIME 2 Forum
- Search results for 'compare classifiers' - QIIME 2 Forum
- Pretrained SILVA 138.1 classifiers (weighted and uniform), reference dbs, and habitat weights
RE 3:
Please see the tutorials: