how can i add more memory to my macbook air, Please!

Whether you can update the drive or the RAM depends entirely on the age of your MacBook Air. What year is it?

You can not add internal storage to your Mac. You can connect an external hard drive if you need additional storage.

If it's really additional storage space you need as KiltedTim suggested and not memory give this portable 1TB drive a look see: 1TB OWC Envoy Pro mini 

OWC is considered by more here to be the premier 3rd party hardware supplier for Macs.

If it's memory you want to increase, again it can't be down on a MBA.

Is it possible to replace the SSD on MacBook Air (13-inch, 2017) model?

FWIW, although it is possible to replace the SSD on certain MacBook Air models, as the others noted it is not possible to increase the RAM. According to your signature you are running El Capitan which is very old and outdated. If your MacBook Air is more than 7 years old it's time to think about replacing it.

Early 2015

according to OWC’s website, yes, that MBA's SSD can be upgraded.

but you may want to find the self install video for your specific MBA to determine for your self if you would be able to perform the operation, or wether you want to farm it out to a professional. I would recommend a professional installation.

Do you want to add RAM memory as in memory sticks or a larger hard drive/SSD. They are two different pieces of hardware. You need to clarify your question. Example to illustrate = RAM Memory https://www.amazon.com/MacBook-Air-RAM/s?k=MacBook+Air+RAM