I have a 2003 Ford f150 and I lost my key what can
You will have to purchase a new key from a Ford dealership. They have all of the key codes for your truck based on the vin#, and can cut one without having the vehicle at their shop. You WILL need proof of ownership for this, most places will require the title, and a photo id that matches who the truck is owned by.
Quick note: I recommend purchasing at least 2 keys so you can have a spare one available in case this happens again.
Ford does NOT keep key codes longer than the last 10 model years. I work for a Ford dealer in the parts department. Your options are limited to... find the lost key, seek help from a locksmith or prepare to replace the lock cylinders and get the ECU reprogrammed. It won't be cheap. ALWAYS have a spare key.
Best thing to do is purchase another lock cylinder with two programmable keys. Call a mobile locksmith and have him come out, install, and program them. As for the doors, you can replace the lock cylinders yourself, but you will have two keys. Unless you want the locksmith to install the door locks and re-key them to one key.