Sounds like you need to bleed air out of the hydraulic system. If you've not done it before, here's how I do it:
Buy some DOT4 fluid, and get some clear tubing that will fit snugly over the bleed nipple on the clutch slave cylinder.
Protect the bodywork with an old towel, and remove the master cylinder reservoir cover (metal cover, plastic insert, rubber diaphragm). Try not to drip brake fluid on any painted parts, wash off with water if you do.
Ideally, suck out old clutch fluid from the reservoir with a disposable syringe and mop out any crud, but don't touch the brake lever until you refill the reservoir with clean fluid.
Put a suitable sized ring spanner over the slave bleed nipple then the plastic hose, and direct that into a waste container. Crack the nipple slightly loose, then hold the clutch lever in, and open the nipple to allow fluid to be released. Close the nipple gently, then release the lever. Repeat the process of lever in, open and close the nipple to let off pressure, release the lever. This will pump fresh fluid down the clutch line from the reservoir and push out any air that is in the line.
You may need to keep bleeding the clutch repeatedly (be patient!), hopefully you will see air bubbles expelled at some point, and the fluid colour will change to fresh lager beer colour. You should also notice the clutch feel coming back to normal spring resistance.
Keep an eye on the reservoir and don't allow it to drop low enough to suck in air, top it up as needed. When nothing but clear, clean fluid is being expelled you are finished bleeding, so carefully remove the tubing, nip up the bleed nipple and fit the dustcap, and refill the reservoir to the upper line, wipe the mating surface clean and refit the reservoir cap.
Dispose of the waste brake fluid thoughtfully, and cap your newly-purchased brake fluid for next time.
