Depends what you mean by 'fairtrade' and it depends on the type of product.
If you are looking at selling goods carrying the fairtrade 'mark' - mostly foods and beverages but also increasingly a range of other products can be 'marked', soap and household cleaning products are a couple which occur to me without putting too much thought into it. You would need to approach The Fairtrade Foundation (in the UK), bring your product or products or potential products to their attention, then the product(s) would go through a fairly lengthy and demanding process of investigation - which also has a cost - expect to provide copious amounts of cradle to grave information about the production of the product, the treatment of the workers, the age of the workers, the working conditions, the opportunities for the workers, etc. Expect any information you provide to be rigorously checked. If your product 'passes' it will be allowed to carry the UK fairtrade mark. Then you start trying to sell it to whoever you choose to approach - supermarkets, corner shops, department stores, dedicated fair trade shops, via a website or whatever using the fact that the product carries the 'mark' as one recommendation for it.
You need to be aware that the mark is awarded on a product by product basis so the fact that your jam has been awarded the 'mark' does not mean that your soap will be nor your vinegar, they in turn will have to go through the same process of investigation and approval. Also be aware that approval is not forever, the product/process/organisation is re-checked at regular intervals. The Fairtrade Foundation does not sell on your behalf though it will publicise the fact that your product has been awarded the 'mark'.
Looking at goods not covered by the 'mark' or looking at other routes to market, you might look to become a member of the World Fair Trade Organisation, you will have to meet their 10 principles of fair trade, and again go through a process of investigation which has a cost, etc etc. Costly but worthwhile if you want to sell internationally as potential customers will check your credentials via WFTO. If you want to sell products just in the UK and are aiming mainly at selling via shops committed to the fair trade ethos then you might prefer to be accepted and recognised by BAFTS, the British Association for Fair Trade Shops. Expect to go through a process of investigation etc etc etc. When you go to someone and say 'this product is fairly traded' they will be checking with BAFTS and/or WFTO (many organisations are registered with both) to ensure that this is the case. The advantage of being approved by such bodies is that they tend to approve your organisation rather than just individual products, but expect them to keep a close eye on your organisation.
How can we ever get started if we have to go through all this first you may ask. Well it is possible to sell products or sell your organisation which are not yet 'marked' or approved by WFTO or BAFTS or whatever on the grounds that you are essentially a start up organisation and working towards such registration/certification/membership. Many dedicated fair trade shops will carry small amounts of products from such organisations BUT they will want to know an awful lot about your organisation and they will keep wanting that information and they will expect to see you actually making the progress towards this, that or the other that you promised and they won't be interested unless your organisation is already a model one in a variety of ways looking particularly at pay, working conditions, opportunities, and age of workers. Don't for a moment think you can spin a tale about good things which don't exist. Folk in the fair trade movement communicate a great deal, sooner rather than later the second cousin twice removed of that nice lady who runs the fair trade shop in x town will turn up on the doorstep of your Indonesian factory, farm, co-operative handicrafts group or whatever and what that person feeds back to the nice lady will then whizz round the globe through the helpful medium of social media.
Sorry if I sound discouraging but I run a fair trade shop and for every genuine approach there are two from organisations which really haven't got the faintest concept about fair trade. Your first step really needs to be to educate yourself about this concept. Look at the BAFTS website, the WFTO website and the Fairtrade Foundation website, read every detail and ask yourself if your organisation or proposed organisation even begins to measure up or if it could be made to measure up.
Also remember that you can't sell anything merely on the grounds that it is fairly traded. It has to be something which people in the country of sale actually want to buy!