Simply listen to the music 'used' in TV commercials and shows, movie soundtracks, those six second internet ads, 15 second ads, 30 second ads, to get an idea of what might sell. Create, get it recorded, Registered for Copyright, make sure you 'own' the Master Recording if you have anyone else involved in Recording it, and be prepared to Supply their Demand if they invite you to submit something.
This is business, commerce, not a hobby, an avocation, but a vocation, not just you, but a company, engaging in Contractual commerce with other companies.
Keep good records, formal files you can reference for future 'uses' of each product.
Titles become important, enabling you to readily identify the product they're talking about and respond quickly.
I had a request to 'Play that Song about that woman.' I had to think about what Song that was, deducing from who made the request and when and where he'd heard it before which Song he was referring to. Professionals in Sync Licensing are not likely to be that vague. They know what Song they're talking about and should be able to specify it by title, and you should be able to find your records and recordings and quickly respond to a Brief requesting it.
Being able to sign a Contract, 'all in', 'clearing' it for 'use', meaning you own the Copyright and own the Master Recording, moves it along toward payday.
Another story from the biz is the band whose 'sponsor' got a Music Supervisor interested in a Recording, who played it for his Boss who wanted it, but...wanted them to take out a horn section. The Sponsor, whose reputation was on the line then, told the band. The band objected to the 'cost' they assumed would entail from going back to the Producer and requesting that the horns track be removed. They ultimately refused.
The sponsor's reputation was stained. The band's reputation was stained. The Music Supervisor had to go back to his Boss and tell them the problem. Bosses don't like problems.
Cost of recording with a Producer, studios, musicians, can be prohibitive. But a Producer might be willing to get paid on the 'back end', after the product gets to market, a check is cut, and the cost is easily covered. Don't let a project stall for any reason, once the product has 'sold' to the Decision-Maker who already took it to his Boss, THE Decision-Maker, who wants your product. The customer is always right. Give 'em what they want. Sell 'em what they want to buy. Your bank will thank you.
Those Sync fees can pay a lot of bills. Cathy Heller's book, "Don't Keep Your Day Job" might be interesting reading for the hobbyist wanting to transition to company, vocation versus avocation.

There will always be another song to be written. Someone will write it. Why not you? www.garyeandrews.com