Hi Sri Ramya,

When you convert a qualified lead, the information from the standard lead fields is automatically mapped to the standard account, contact, and opportunity fields. Please reference "Lead Conversion Mapping" in the Help & Training to see the mappings of standard Lead fields. 

If your organization has custom lead fields, you can choose to map them to a custom field you've created on the Account, Contact, or Opportunity. Standard fields cannot be mapped to custom fields, nor can standard field mapping be modified. 

To specify the mapping for custom lead fields: 

1. Click on: 

Setup | App Setup | Customize | Leads | Fields. 

2. Click on "Map Lead Fields" (At the bottom of this screen). 

3. For each custom lead field, choose a previously created custom account, contact, or opportunity field into which you want the information inserted when you convert a lead. 

4. Click "Save". 

Remember to map custom lead fields to other custom fields of the same data type, that is, map numeric lead fields to other numeric fields. 

Also, remember to map custom lead fields of type number, currency, or percent to another number, currency, or percent fields of exactly the same length and decimal places. For example, if your lead currency field has a length of 3 and 2 decimal places, map it to another custom currency field with a length of 3 and 2 decimal places. 

NOTE: If you change the data type of any custom field used for lead conversion, that leads field mapping will be deleted. 

I hope it will be helpful.

Please mark it as best Answer if the information is informative.

BestRegards
RahulKumar