Hey mate! Well, first off that's a tall order you've got there - not only is that army illegal to play (unless you're using unbound), but it's not all that competitive. Correct me if I'm wrong, but it has;
1. a captain with a power sword and storm bolter
2. 10 tactical marines with a flamer and heavy bolter
3. 5 terminators with power fists except for the power sword sergeant
4. a dreadnought with a multi-melta
Is that correct? I'm going to assume it is. Unfortunately, if you want to play competitively you're going to have to expand your force a bit. There are a few different routes you can go.
First of al, transports are extremely important. Whether you're dropping your force into the heart of the enemy or driving them up in rhino transports, you need a way of getting them to your opponents. Or, at the very least, rhino's can block LoS for your marines if you want or create choke points.
Second, you're going to need troops. A battleforce (if those still exist) is a pretty good deal, but essentially you need either more tactical marines or scouts.
For more on army building, check out our space marine lists
section as well as the space marine tactics
As for tactics for playing your specific force, if you're not going to get new models then I suggest the following. Keep your dreadnought hidden from LoS of enemy heavy weapons as much as possible - he's super vulnerable to it with only av11 (or is it 12?). Split your tactical squad into combat squads; the missile launcher and 4 bolters on an objective and the sergeant flamer and 3 bolters to take a forward objective. Then hide the terminators similarly to the dreadnought and wait for opportunities to shoot and assault things you can kill. It's hard to say exactly what you should do as you have a very non-optimal force, but oh well.
I hope this has helped, but I'm going to leave the rest to the more experienced smurfs. Cheers mate!