Are you circumcised? Have you ever had an indwelling catheter before?
This is often caused by the hole you urinate out of being small (meatal stenosis as we say). If this is the case, it can be easily dilated in the office by a urologist.
If your are not circumcised, it is obviously important to pull back the foreskin. If you can not pull it back all the way, this may also be a cause.
Hope this helps.
Please let me know if I can help further.
Having had a catheter before is a risk factor for a urethral stricture (narrowing in the tube you void out of).
Do you have trouble starting, a weak stream, or a sense of incomplete emptying of your bladder?
I think the easiest thing to do is to see a urologist. They could place a small telescope inside your bladder (cystoscopy) and evaluate your urethra.
Although this may be obvious, in the meantime it may be helpful to just sit as you void so you dont ruin your clothes!
Best of luck,