Well, that's slightly unusual for all 5 strings to be the same length.

Presuming you're right handed, when you hold the banjo in your lap the uppermost string (toward your chin) is called the 5th. Next one moving down toward your lap, is 4th, then 3rd, 2nd, 1st. USUALLY the 5th string is much shorter than the others, with a tuner located beside the 5th fret. In a typical set of 5 string banjo strings, the 5th string and the 1st string are the same gauge, at either 0.009" or 0.010" thick. The other strings in the set will be of appropriate gauges.

However on your banjo (does it have a brand on or in it anyplace that you can read?) you will need to have a small device just behind the 5th string 5th fret to permanently "note" that string for the proper pitch to play 5 string banjo songs. Some would call it a 5th string capo. Cheapest and most effective is a tiny nail from a model HO gauge railroad set. You hook the 5th string under it to permanently "note" the string at the 5th fret. You can also stick a short piece of plastic under the 5th string at the 5th fret. You can buy a thing called a "Reagan capo" the screws on to the string itself to do this. A quick DIY rig is the plastic removable cap to the cheapest ball point pen. This cap usually has a short spear on it to hang in your shirt breast pocket. Jab that spear beneath the 5th string right over the 5th fret until it permanently raises the 5th string as if it were being noted. Cut off the part of the spear that's too long and interfering with the 4th string.

Now you're ready to tune up. Tune your 3rd string to match the 3rd string on a guitar. That note is G. Tune the 5th string to be one octave HIGHER than the third string. The first string is tuned to D between the notes of the 3rd and 5th strings. 2nd string is bit lower at B. 4th string is D an octave lower than the 1st string. When all 5 strings are retuned and "touched up" so they're correct to a piano or electronic tuner or guitar, strum across and you have a pleasing open G chord, without using any left hand fingers at all.

From this point find any "beginning 5 string banjo lesson" on line and you can start learning left hand chords and how to strum or pick the strings. What kind of banjo music interests you? Bluegrass? That style is played with thumb and first two fingers of the right hand and is called "3 finger picking". Old time music which is usually led by a fiddle, is played with thumb and first two fingernails of the right hand in a style called variously "frailing", or "clawhammer" style. There are beginners lesson videos on line for this style too. Be prepared, for old time music, lessons may tell you to immediately return one or two strings of the banjo -- away from that pleasing open G chord. At no time will you expected to fret a note on the 5th string lower than the note it is producing automatically at the 5th fret. For all purposes it's as if those 4 frets don't even exist under the 5th string.

Can't wait to see photos of your banjo with 5 strings of same length. This might indicate it is very very old, or a specialized type of 5 string banjo that's out of the ordinary.

I have seen one such banjo with 5 equal length strings at the Franklin D Roosevelt Mansion museum in Hyde Park NY. It dated from the 1920s I believe. Built by the Vega Co. of Boston. Unusual.

One more thing, tell us where you are located.  If not in the USA, it may help explain why your banjo has 5 equal length strings, which would lead to better advice for you.

Are we sure it's not some breed of tenor/plectrum banjo with an additional string? That would be my assumption unless there is evidence of the 5th string having been spiked/capoed.