If you want to take piano lessons, then a good acoustic piano or a good digital piano is necessary. A digital piano has keys that are the same size as a regular piano, has all 88 keys, and is not the same as a “keyboard” or “synthesizer”. A keyboard or synthesizer has a very different touch than a piano, and if students begin lessons on these, they will soon become frustrated. Playing piano requires strength in the ligaments and tendons in the hands to press down the keys at different speeds. If you learn on a keyboard, whose keys require virtually no strength to press down, you will have difficulty when playing on an acoustic or digital piano.
An ideal instrument is an acoustic grand piano, but I understand that is not always possible – especially when you may not be sure you will continue your piano studies to make it worthwhile! I recommend that you get the best piano you can afford, because with most things, you get what you pay for. Many local music instrument stores offer digital piano rentals. Digital pianos often take up less space than most small upright pianos, and these are ideal for apartments or condos where silent practice may be necessary (either by turning down the volume or using headphones). Many of these stores allow you to rent for a period of time and then apply your rent payments toward the purchase of a piano when the lease is done.
If you must use a non-weighted keyboard, just understand the limitations and do the best you can with it until you are able to upgrade.