That's beautiful! Why do you need a long exposure time for photos like these? Does it make it more colorful, or is it to keep the ISO and noise lower?
A few thoughts:
1) You might find the book "Understanding Exposure" by Bryan Peterson to be of help. Further the series of 4 books by Scot Kelby called "The Digital Photography Book (1 through to 4) might also be of help.
2) Keeping the ISO low is NOT always the best way to get the most noise free photo. Now whilst it is true that if you increase the ISO the amount of noise also increases; it is also true that if you underexpose a photo then you will get more noise when you brighten it up in editing, than if you'd have correctly exposed it using a higher ISO value.
"Keep the ISO low" is a mantra often taught to those new to photography and its bad advice in general. Because it makes you focus on keeping the ISO low when it might not always be the best option. If you've already set your aperture and shutter speed then if the ISO needs to be higher to get the shot then chances are its best to raise the ISO above all else.
As you gain more experience you'll come to understand this better and be able to make that judgement call of when you can drop the shutter speed or widen the aperture instead of raising the ISO (or vis versa).
3) Use a tripod when you have one and when you need one. In theory most shots will benefit from one; although in many cases the benefit might just be reduced fatigue on yourself.
Several examples of common situations where a tripod is often critical have been made above including:
a) Long exposures - when you want to do a shot using a very slow shutter speed. Because then you need a stable platform to shoot from otherwise all you'll get is the blur as a result of handshake.
The rule of thumb posted earlier about minimum hand-shake shutter speeds is a good one to go by, but remember if you pose yourself properly - if you hold the camera right - if you lean against something firm (including the ground) - all these tricks can help you get an extra few shutter speeds before handshake blur is a problem.
b) Single position shots - if you're going to be doing something in a single spot for a long period of time a tripod can take a lot of weight off your arms. This might be waiting in a hide for a shot - it might be setting the camera up to take a long series of studio shots (eg copy work or photographing products). Landscapes are also popular as you're sitting there waiting for the light and then often might ahve a slower shutter speed too.
c) To help with stability in general - big lenses are often heavy and thus will fatigue you faster (which impacts your ability to handhold significantly) so a tripod to take the weight of a heavy lens is a big boon. Similarly if you're doing macro you might find it of help in some situations (you can do macro fully hand-held if you want or you can use a tripod - choice is yours).
d) For precise composition - sometimes you want the composition just so - to be just as you want it. This might be on a landscape - a person - a flower - a bug etc... Whatever the situation the tripod is there to help you.
NOTE higher end tripods have a tripod head that removes because what makes a tripod work isn't just hte legs but the head attached to it. Different heads are suited to different things and some are useless for others (eg geared heads are generally no good for any action based photography or situations where you're moving the head around fast).
Lower end tripods have a fixed head (generally speaking) which will typically be a 3 way head or ballhead in some cases.