Should I change my guitar strings?

Question:I play acoustic guitar,
3 nylon strings, 3 metal ones.
I teach myself & am a beginner. only started playing 3 months ago & I don't know if I should change my strings.
I play every day for 15 min-1 hour.
Except for 2 separate weeks,
when it wasn't really possible.
I tend to fingerpick,
as I play late at night and it's quieter,
but play with a pick perhaps once a week.
Honestly, I lost my electronic tuner,
& can't tell how in tune my guitar is.
It's my brother's old guitar.
He never really played it,
and then gave it to me a year and a half later.
He always kept it in a case,
but since I play every day,
I just keep it propped against a wall.
I have a lot of trouble tuning it,
but I don't know if that's me or the guitar.
It just doesn't sound very good.
& maybe it's my imagination,
but the nylon strings appear to have white areas where the string gets pressed against the frets.

*continues, one minute*

Best Answer - Chosen by Asker: I play acoustic guitar,
3 nylon strings, 3 metal ones.
I teach myself & am a beginner. only started playing 3 months ago & I don't know if I should change my strings.
I play every day for 15 min-1 hour.
Except for 2 separate weeks,
when it wasn't really possible.
I tend to fingerpick,
as I play late at night and it's quieter,
but play with a pick perhaps once a week.
Honestly, I lost my electronic tuner,
& can't tell how in tune my guitar is.
It's my brother's old guitar.
He never really played it,
and then gave it to me a year and a half later.
He always kept it in a case,
but since I play every day,
I just keep it propped against a wall.
I have a lot of trouble tuning it,
but I don't know if that's me or the guitar.
It just doesn't sound very good.
& maybe it's my imagination,
but the nylon strings appear to have white areas where the string gets pressed against the frets.

*continues, one minute*

if you have nylon strings then you have to buy nylon ones, too. Putting on metal ones would be like ...mmm well, a bad thing. They don't cost much and if you feel like changing them, then it's the right time. Especially if your brother never changed them and it still has the original strings (usually not very good). Some people change strings after every 2 or 3 months. I have had mine for 3 months now and i am already thinking of putting on new ones.
If you go to buy new strings, then don't take the cheapest ones. In my opinion the sound is better with better strings. So if you pay just a bit more, you will get much better results=)

yes, and it's better when your strings are metal

i think it's only $ 0.25