I can suggest two main probable causes:

1. Worn-out synchros
2. Clutch issue:
a. The clutch is not being pressed down deep enough
b. If it's cable type, it may need some adjustment
c. If it's hydraulic, check for any leaks, the fluid level, and if there's a necessity to bleed the system. It could also mean that either the master cylinder or slave are blowing fluid past the pistons

To confirm item 2c, try to pump the clutch a few times and see if that will get you into 1st gear much easier.