This is actually a common problem. There is a reverse detent plate under the shift boot to stop you from pushing the shifter too far to the left and accidentally selecting reverse. Try pulling the ring on the shifter, pushing the shifter to the left as if you were going into reverse and then, while holding the ring, push the shifter back toward neutral and up into first. Smooth engagement of first?

Pull the shift boot and you will see the adjustment bolts. Move the plate to the left and then try selecting first....keep adjusting the plate to the left until the shifter goes into 1st without protest.

Other cause could be that your clutch is not disengaging completely. Check the cable adjustment, if the cable is adjusted properly then you may have a dead pilot bearing. Which means that, unfortunately, you will have to pull the clutch and do a clutch job.