Salesforce offers a free of charge, fully functional instance called a Developer Organisation, the only real restriction of which is limited data and storage capacity. Anyone can sign up for one and do as much development and make as many changes as they like, making it a great learning environment or place to work on new ideas.
Signing up is easy, simply navigate to developer.salesforce.com and fill out the form.
When you sign up to a developer org you’re asked to enter a username as well as an email address. These can be the same but they don’t have to be, in fact it’s often better if they’re different. Your email address is just used for sending you emails, whereas your username is used to log in, which means using a username different to your email means you can sign up for multiple developer orgs; it’s perfectly normal to do this, Salesforce won’t have a problem with it at all!
So I strongly suggest you use your usual email but create a unique username.
Salesforce will send you a confirmation email with a link to set your password, so make sure you have access to your email. Once you’ve set your password you can login at login.salesforce.com.
Salesforce have put numerous limitations in developer orgs to make production use impractical. Some of the key limitations are:
There are a number of other limitations which can be found on the Salesforce website here help.salesforce.com
You also won’t receive official technical support, although you may be able to get help through the community message boards.
Even if the limitations above are acceptable the developer org license prohibits commercial use, so no - it’s just for non-commerical purposes.