I am often asked for recommendations for learning ASL, and above all, my recommendation is to be friends with people who use ASL in their daily life. I initially learned ASL through the Language Learning Program at UC San Diego, and have continued to improve by signing with colleagues and friends, hearing and deaf.
If you aren't friends with people who can sign, then it's important to ask yourself why you want to learn. There are plenty of good reasons for wanting to learn ASL. But I also often see well-intentioned hearing people act like they consider ASL to be a fun diversion, which makes me very uncomfortable.
That being said, if you have some relationship to the ASL signing community, and you're aware enough that you don't want to constantly impose on your signing friends, but you still need a bit of help to improve, then I've definitely got some recommendations for you!
The Basics
Lifeprint is THE go-to for learning ASL online, and has been since the late 1990s. There are a bunch of short posts written about ASL structure/use/culture, and an associated YouTube channel with lots of videos of teacher-student interaction.
Since the mid-2000s, ASL That! has risen as another great online resource. There are lots of videos of sign examples and lessons and discussion, and the discussion continues on the associated Facebook page.
Another great resource is Handspeak, which is run by a poet and storyteller who is a signer of Canadian ASL. Many longer examples and artistic works can be found on the associated YouTube channel as well.
If you're just looking for a particular sign, SigningSavvy is also a good dictionary resource. There are a handful of other ASL dictionaries online, but I haven't personally found them to be very useful or easy to navigate.
More Advanced
Convo is a Deaf-owned VRS company that also does lots of community-centered work. Their YouTube channel has several high-quality videos with clear signing.
In the past few years, ASL news broadcasts have become increasingly popular. These are a great way to keep your receptive skills sharp. Here are a few I like:
The Daily Moth is just great, with many videos on their YouTube channel.
Deaf Newspaper has been posting interesting videos for over a decade.
The Deaf Performing Arts Network posts news stories on their Facebook page.
Deaf-Hearing News is now defunct, but their 2013-2015 videos are still up.