First off, get rid of the circle/lighting bolt, that picture does more harm than good for reasons I've outlined before. other than that the rest of the tats are fine.

Your line work needs work, its very plain and straight forward, mostly consisting of singular lines, which is fine if you have detailed color, and your not bad with colors in terms of tattoo art, but they are still pretty straight forward in your art. But that's to be expected to a certain point, as tattoos often cannot have super detailed colors.
What You need to do is actively study tattoo art, and you probably are, but rate your work against it and come to your own conclusion. If I were a pro Tattooist I'd not take you on just yet. You'd need to show me a few things first, that you can blend in both black and in colors. your work with solids is commendable, but most tattoos worth more than a few bucks have blends. So see what you can do to learn that.

The best way to show your worth an apprenticeship is to bring something to the table any regular Joe cant just waltz in with. Anyone can copy a stencil. but artists can offer more than that. they can customize and build a persons dream tattoos from scratch. My sister is madly in love with the custom tat she had designed for her. she brought in three reference pictures and the guy came up with a master piece.
So obviously you have to hone your art skills. but also your creativity, hthat is actually far harder to hone. I'm a concept artist, meaning my art has to show something not seen, something with a pinch of originality. I may not be great at it but the more I do it the better my ideas are.

Yes You do need to learn the basic common art used in tattoos as Ive outlined before, but that's for in case someone comes in with a request that uses those basic features, That way you can fill the request. so keep working on those because they are the roots and many requests will come of them.
But when you present YOUR PERSONAL WORK to be sold to a customer, You need to show them something they could not imagine, something remarkable and original, that makes them drop that request picture in there hand and by your art instead.
Great concepts can contain the same typical elements as found in most tattoos, but have a style or a lay out unseen. For example: My Werewolf style is very uncommon, and it tends to attract people, even though a werewolf is not at all original. its one thing to draw a skull, its quite another to draw one like nobody has ever seen before, that is what sells tattoos, and what will sell you as a Tattooist.

Now this may just be me running home to my own roots, but if you want original line and color art you CAN apply to tattoos and is out of this world chock full of picture concepts, Then to start with Id recommend you go out and getting a stack of comic books, and observing how line art masters do it. Comics must show different angles and perspective ten times a page, and in stunning detail, very few tattoos have comic book style lining, mostly due to the fact the style is not common to tattoos, but also because comic style line art is by far the hardest to learn. Master that and you may have something unique to offer the tattoo world.
But even if you don't use it directly, it will teach you to vastly improve your works in one way or another. then simply apply that knowledge to the common formulas found in tattoos and you'll have a head start.