Here are the easiest 3 steps to started :
1. Create login : https://developer.salesforce.com
2. Create Login : https://trailhead.salesforce.com
3. Access your sandbox : https://login.salesforce.com
Finally the developer sandbox, this login is for your salesforce actual handson experience.You wil use this a lot.
I will recommend you to create all three logins and bookmark these links for future reference.
Cheers see you in next post.