There are various threads scattered around, but off the top of my head I'd say:

• You will want an easy way to top up tire pressure before riding. Floor pumps are a good choice, provided you have a place for them, quick and easy. Especially if they have a built in gauge. I picked up a Topeak Joe Blow Sport at an REI Garage Sale and it's been great.
• Stuff to handle a flat tire while out. Pump or CO2 is your choice, both get the job done. See Build a Bike Patch & Flat Kit | The Sweethome
• Half gloves; they add a little more vibration damping and more importantly if you do take a spill they help prevent road rash on your palms. Not expensive, you can get decent ones on sale for under $15.
• Helmet -- up to you if you want one or not, I personally prefer to ride with one. Good, well ventilated, comfortable helmets can easily be had for $60-70. There are whole threads on the topic so I won't go further here.
• Consider eye protection even when not wearing sunglasses. A bug in your eye at 25mph sucks. Clear safety glasses work well and are cheap; search Amazon for 3M products.
• If you'll be leaving your bike unattended, even for a minute, you'll want to lock it up. Again, whole threads devoted to the topic. In summary, all locks can be defeated; just a matter of how badly the theif wants your bike. Scale the level of locking to the situation.

I'm not familiar with that Cateye computer; I'd just suggest thinking about the future when selecting something though. With any speed or cadence sensors I'd choose ones that support both ANT+ and BTLE (Bluetooth) as they'll then be compatible with a wider range of GPS cycling computers and gps multisport watches as as the ones from Garmin and from Polar.