Start with the rules. The City of Ottawa's fence by-law sets maximum heights, and the limits for front yards are lower than for back and side yards. Corner lots have extra rules to keep sightlines clear near intersections, and pool enclosures follow their own by-law. Checking all of this before you design saves you from building a fence you later have to cut down.
Next, call before you dig. Ontario One Call arranges free utility locates, and no post hole should be dug until the lines are marked. On rural lots, septic beds and well lines need to be located too.
The posts decide how long the fence stays straight. Ottawa's frost reaches deep, and a shallow post gets lifted a little every winter until the fence line waves and the gate stops latching. Posts need to be set deep, plumb, and on a straight string line, and gate posts deserve extra attention because they carry the most load.
Then choose the look. Classic wood privacy fences are affordable and easy to repair. Modern horizontal slat designs pair well with interlock and contemporary homes. Whatever the style, good hardware and a properly hung gate make the difference between a fence you enjoy and one you fight with every time you take out the garbage.
Finally, talk to your neighbours. A shared fence goes much more smoothly when everyone agrees on the line and the look before the first post goes in.