For a plasmid PCR, usually 10 ng template are enough.

precipitate your dna and resuspend it in lower volume.

300 µl plasmid

+700 µl 100% EtOH

10 µl 3M NaOAc

Mix and spin at 4°C for 30 min, max speed.

wash with ~500 µl 70% EtOH and spin for 5 min, max speed.

let the pellet air-dry and resuspend in a low volume, nanodrop and bring it to the concentration you desire.

I mean, if use plasmid, how can I prepare 10 ng? How much water and plasmıd should I mix ?

Let's say you need 10ng for one 50ul reaction. So, you can prepare 30ul of plasmid at concentration of 10ng/ul.

Eg:

Ur plasmid = 100 nanogram/microliter (ng/ul)

Concentration required = 10 nanogram/microliter

Use M1V1=M2V2 formula:

100 (Volume 1) = 10 (30ul)

Vol 1 = (10 x 30)/100

= 3ul

So, that means you need to pipette out 3ul from ur stock plasmid (100ng/ul), add with 27ul ( 30ul - 3ul) (coz you want to prepare 30ul of working stock). What you have now will be 30ul of working stock plasmid with concentration of 10ng/ul. So, for each 50ul pcr reaction, juz pipette in 1ul of this working plasmid stock.