Start with Garage sales and Goodwill stores. Take some time to look around, buy a couple of things even if you lose a little. Search eBays completed listings and get a feel for what is selling and for how much. You might be surprised what sells.
Certain plastic soldiers sell well as do baccarat glasses. At least when I was selling collectibles. Certain records sell well but sometimes not the ones you think. They have to be rare. I sold an Arthur GUthrie 3 record set for 175.00 to England. Stay away from any type of sports cards unless you know one is very rare. An AROD rookie card will sell for 4$ unless its the right one which will sell for 200$ And an Elvis 45 can sell for 500$ or more if its 1st press otherwise 5$.
I had 3 Bob Seger records and only one sold because it was a rare concert pressing.
Costume jewelry only sells in lots of 20 or more but interesting pieces will sell for 5 to 10$.
Bags of legos will sell. If its a big bag you could get over 50$ bucks.
Silver pieces sell pretty good. I had a lot of silver jewelry and sold in singles and lots. They went pretty good.
Anyway, it's an idea. Hope it helps.