When writing dialogue all the characters sound the same. I'm not sure I even know what giving a character a voice means, but my dialogue sucks so any advice will be greatly appreciated.
A character has a "voice" if, when you read about a page worth of that character's dialogue (without any attributions) you could identify the character correctly.
Not EVERY character needs a voice; in particular the above test suggests, if they don't even HAVE a page of dialogue in the book, they don't need a particular voice.
One of my warriors has a propensity for describing things, including emotions or pretty things, in militaristic terms. If his mind is changed by an argument, he wouldn't say "I agree", he says "I surrender" to the argument.
Another character was raised in a "traditional" part of the world, and speaks a little archaically; if you see an explanation including the word "thus," where you would normally use "So," this is the only character that does that.
You can use emotions, too. A character may grow angry speaking on certain topics, even though the conversation is benign.
Some characters are deeper thinkers, and use longer compound sentences. Some speak very simply. I personally don't stray into either accents or fancy words, but a scientist is likely to think with more complexity than a janitor. The scientist may speak longer sentences with qualifiers; the janitor can speak in shorter sentences.
A character might have a tendency to interrupt themselves, or repeat themselves, or their mind keeps running after they say something, and they say "And another thing" or "you know what else" every once in awhile.
You do not have to get heavy handed with this; their "quirk" doesn't have to be present in every sentence they utter. That gets grating.
Ultimately, if you are doing your job of imagining your characters and their personality, their dialogue will be a function of that. Where they were raised, their "class" or economic circumstances, the slang used there, how much cursing was going on, the things that entertain them, what they do for a living, who they interact with frequently (family, friends, coworkers) and what THEY do. That is how we learn to speak, it is where we learn our metaphors (is it a touchdown or a home run or a slam dunk?) and it is difficult to shake without an effort.
It also doesn't have to be a repetitive 'catch phrase' or repeated use of a special word. A character can have a propensity for "comparison". Nothing is 'sweet', it's 'sweet as clover honey.' It's not difficult, its 'hard as threading a needle with your eyes closed.'
Or a character might get lost in what they are saying, differently each time, but occasionally lose track of what they were saying and trail off, or start over. Or not realize it until whomever they are talking to points it out.
Giving main characters a voice helps the reader keep them separate, and (along with conflict) this helps sustains the illusion that their longer conversations are not all just the author talking to themselves.
When it comes to crafting individual character voice, be aware that there are existing lists you can find that will give you specific items to think about. These lists include things like: Education level, dialect, age, station in life, accents, loquaciousness, and so on.
You can add a catch phrase for sure, but make it a harmless and invisible one. My characters do each have a catch phrase--and no one else in the novel is allowed to use it. (but that is only one part of their voice.)
Example: One and only one of my characters is allowed to begin his sentences with the word 'Look.' Another uses "Well," and a third sticks the phrase 'of course' into her dialog on a regular basis. They each came to their personal verbal tics naturally and logically. The scientist character quantifies his observations with the phrase 'a bit.'
Look, it's just something to become aware of. I mean, of course you're probably already aware, but if not, pay a bit of attention to people around you. Well, that's how I see it anyway.
Here's some de novo dialog with very little attribution. Can you get a sense of character voice? Why?
“Are we there yet?” she whined.
“Sweetheart, shush. Your father needs to focus on driving.”
“How much longer?”
“Would you like the iPad?”
“I don’t want the iPad!”
“Honey, settle down. We'll get there when we get there.”
It has to do with how characters treat one another. It has to do with punctuation. It has to do with the words they choose--simple or advanced. "Would you like" is a more sophisticated way of saying "Do you want."
It has to do with all sorts of stuff--and you can make yourself a formal list for each character if that works for you, or feel your way more organically. I like lists, personally. Eventually it becomes second nature to write in a character's voice.
Find yourself a writer's group where you critique each other's work. Or a group of friends/family will work for this. Make sure it's in person (or by video/phone conference, if you must). Now, read your work out loud. Not the whole thing; maybe 10-15 mins.
Even if you're writing work meant to be read individually, using the technique of reading it out loud really helps you, the author, figure out where things aren't working right. While this technique works for all kinds of things, where it really shines is in figuring out dialogue problems.
When you say your dialogue sucks, that tells me that maybe it doesn't sound real. So that's your first step: making sure that all the dialogue sounds like it could have come from the mouth of a real person.
Next, you want each character to have a unique voice. Some will really sound different and, for others, the difference will be subtle. And that's okay. Think about in real life how out of a group of 10 friends, several will (or could) respond in the same way to something. Make sure the main characters are really different and aim for some slight differences among the others.
If your critique group is willing, ask them to pretend to be a character and react as that character would. Do some improv. If they get it wrong, how is it wrong? Ask the other members of the group to weigh in here.
Remember too, it's not just what someone says or how they say it. If they speak is part of their voice too. Sometimes people respond with grunts or nods. Others selectively ignore what they don't want to discuss. Some will answer if they're alone but will let others answer for them if they're not. Then there is body language.
Once you're able to see each character as a full, complete, person, you will hear their voice and see how they interact with others. Then you can call on these things as you write for them.