Hi Alice,
I apologize for the bluntness, but without this piece of information is difficult to properly reply with a response potentially useful to your expectations: what's your relationship with wine?: "an aficionado that wants to take advantage of the fact you're visiting wine-country to have a winery-experience because it's difficult to do so in my neck of the woods"... or "a picky connoisseur that has visited many wineries and is looking to taste new wines on-site"??
If the first, I'd recommend a visit to Codorniú Estate in Penedès County, one of the oldest wineries on these shores, dating from 1551, with a yearly turnover of 300 million euros; or to Freixenet, another mammoth producer that bottles over 200 million bottles each year. Both are aimed at first-timers and one visits the 'caves' (underground cellars) on a guided tour and has a taste at the end of the tour. Prices are around 20€ pp. I believe reservations are needed, check their websites. You can visit on a day escapade from Barcelona by train as they're at are less than 1h from the capital-city. In Codorniú you can opt to a visit with lunch (~under 50€). Both experiences are similar, but I personally like better the first one.
If the latter, http://act.gencat.cat/wp-content/uploads/2017/11/ENO17_ANGLES_Baixa.pdf where you'll find many options to visit smaller wineries where you'll be able to have a more intimate experience getting to know the owners of the place. For many though you'll need to drive.
Enjoy!