4e is a very tightly balanced game, and you generally shouldn't need to modify monsters to make them more difficult as they typically provide sufficient challenge when used properly. The following is some advice to make sure you are pursuing in order to provide balanced encounters:
Monsters almost never appear alone.
4e is a squad combat game. Rarely, if ever, should your PCs fight a single monster. Make sure that most fights are against a similarly sized squad of standard monsters (or slightly fewer elites, a single solo and some minions, or a mix of all of them).
The most important things to keep in mind are the following:
- Encounters have XP budgets, go find the table in the monster book you have. Find your PCs' level and make sure you are building your encounters to the appropriate budget
- Use monsters of the same, or slightly higher or lower than your PCs. Don't go too high, and don't go too low.
- Generally 4e is balanced around 4-6 encounters per day. If you're PCs aren't doing that, then make sure you occasionally send them to a dungeon where they can't rest easily.
- Make sure your monsters are up to date. Monster math got a huge overhaul in Monster Manual 3, so make sure that you are using the damage expressions from that book (and HP/defenses numbers from there too). Both Monster Vaults and the DM kit use this math as well so monsters from there are good. If you're not using this it could explain some of your problems.
Also, it's really important to make use of the environment. Make your dungeon rooms interesting, add traps, or things that need to be interacted with along with the monsters in the room. This is an important part of keeping yourself from getting overwhelmed in later levels where monsters are much harder to run.
Lastly, don't fret too much if they are wiping your monsters, that's something of the intent of 4e. If they are doing too well, then you might add an extra standard monsters worth of opponents to some fights, but I would not tweak too much.
(very last thing, since you're new and 4e is unendingly complex, it might not hurt to make sure your PCs are built correctly and that you aren't accidentally making them more powerful than they should be, believe it or not this is fairly common).
Party specific advice section:
You've got a pretty solid party there, but it's not balanced. It's going to have a couple of big glaring weaknesses.
You don't have a tank in this party, but you do have some tough guys. there isn't a defender here to dish out marks so you should be able to move your NPCs around fairly easily without the typical reprisals. Choose enemies with tons of shift powers to slice and dice their way through the front lines of cleric and warlord (and 'lock if it's con-lock) to the squisy ranged ranger.
You don't have a true controller here, make use of minions. Some of these classes can minion pop as a secondary, but with no true controller, floods of minions may well challenge this party more than it would a balanced one.
Use brutes. One of the reasons they are taking down your monsters so effectively is that you have 3 characters who (if well built) can wipe out half or more of a single monster's HP in a single shot. Brutes have tons of HP and lower defenses and damage, use them to soak some of the strikers' body blows and make your other monsters more effective overall.