If the domain expired just for a "few days ago" you can't register the domain right away. The domain is placed in a kind of "quarantine" for a month or so. It depends what kind of top domain it is as far as i can remember. The reason is - if the old owner just has "forgotton" to pay for an other year the owner has an other month to pay before the owner lose the domain and it's free to register for someone else.
You will have to wait around 60 days from the day it expired (for .com) before it is available to be registered - other extensions vary in process/days. During this time you can either attempt to contact the current owner to see if you can strike a deal with them to renew then transfer it over to you, or you can try one of the drop catching services or you can attempt to drop catch it yourself. Drop catching is the process of timing the exact point the domain will be released for registration and attempting to register it the second (millisecond!) that it drops. Depending on the quality of the domain name, you could have hundreds of people/companies all trying to catch the same domain.
GoDaddy expiring auction & closeout domains. You'll also preserve the domain registration date, I got a 11-year-old domain from GD.
if it is a good domain the best chance to buy. you find out domain Whois, then you contact owner domain or contact administrative contact where domain registration . You can also acquire domain at domain auction marketplace.
You'll need to give it some time, domains aren't available straight away once they expire. Give it a couple of weeks then you should be able to buy it. If someone else gets it, you can contact them through their registrar and buy it off them.