Schema.getGlobalDescribe().get().newSObject() can be used to convert String to sObject using Apex in Salesforce.

Sample code:

//Setting String str as Account
String str = 'Account';
//Casting String to sObject
sObject obj = Schema.getGlobalDescribe().get(str).newSObject();
obj.put('Name', 'Test');
obj.put('Description', 'Testing');
system.debug('obj is ' + obj);

Output: