Well, as for "accessories"

Must Haves

1) Helmet - you have this. Good for you.
2) Saddle bag and pump, tire irons, spare tube or patch kit.
3) Water bottles at least 2 or hydration pack.
4) Shorts
5) Identification somewhere. In our club, we have had 2 of our members (who were out riding solo) found stunned without ID and that is not a good thing.

Nice to Have

1) rear blinker - one of these days you will be out in a low light situation. These cost something like $12 and I get the kind with AA batteries because the run life with AA is something over 100 hours.
2) Cycling shoes. You can use shoes with toe clips or you can go with clipless pedals.
3) Clipless pedals - much better transfer of power. In your case, SPD would be nice and some can be bought pretty inexpensively.