My multimeter only has current measurement up to 0.25A, but I need to measure currents up to 10A.
What are the options I have?
Can I use a resistor to reduce the current and measure?
Is there any software to calculate a resistor that I should use?
Please take a look at my multimeter.

 

A series low value resistor is needed. You will have to calibrate it but just use a bit of steel wire (fencing wire or coat hanger) in series with your load as a resistor and measure the voltage across it.

 

A series low value resistor is needed. You will have to calibrate it but just use a bit of steel wire (fencing wire or coat hanger) in series with your load as a resistor and measure the voltage across it.

I want to measure current not voltage

 

I want to measure current not voltage

Yes!
You measure the voltage dropped across the series resistor and work out the current.
Inside a MM on a 10Amp range, there is a thick bit of wire (current shunt) that the current goes through and the voltage across that is measured.
https://hackaday.com/2018/02/08/how-current-shunts-work/
Watch the video at the end

Just remember, he is after "comedy" so please do not do the stupid things he does, like directly connecting to his Variac. Most Variacs are not isolated so that is potentially lethal!

 

One candidate for 10 Amps may be considered is a bicycle spoke.
If it is 2,2 mm thick and 30cm long then it`s resistance, using the (1...1,4)E-7 for steel, is ca 0,08...0,1 Ohm, thus at 10 Amp there will happen 0,8 to 1 Volts easily measurable voltage drop.
Calibration of course is needed at the place.

 

Looking at the picture of your meter where the voltage range is marked in mV it shows 2.5, 0.25 and 0.1 I suspect these are volts rather than mV so they really are 2500 mV, 250 mV and 100 mV. If this is the case you could buy a current shunt such as this one
These are just resistors with a high current rateing. You would just connect your meter set to the 100 mV range to the shunt and pass the current through the shunt. You will notice that there are two connections on each end of the shunt. Connect your meter lead to one teminal and the wire carrying the current to the other. This is done to eliminate the effects of contact resistance. Your meter will read 75 mV with 10 amps passing through the shunt so you can divide the reading in mV by 7.5 to give the current in amps. This is just what the other answers have described but it is an easy way to get a low value resistor that can carry a high current. (This shunt will have a resistance of 0.0075 ohms.)

Les.