This is gonna be long... but I enjoy helping people save time and money as they first get into the hobby
First of all, you should start with a small force (1000 points), and tune it to your play-style. I would recommend that you invest in some additional transports as you determine what troops you prefer to field. If you like how the scouts perform, pick up a land speeder storm. If you like the Tacticals better, add another Rhino, etc.
Anyway, as for the models you have, here's some things that Imperial Fists can make good use of:
Scouts - All bolters, all day long. You get Bolter Drill (and "Close Range Bolter Drill" if you're playing as the 3rd Co. in the Sentinels of Terra supplement) which helps counter the fact that they are BS3. You can probably salvage some bolters from the other kits if you're a few short (I think they might only give you 4 because they also include shotguns?) I think very quickly you will like how they do and will be investing in a Land Speeder Storm. (If you do, and you're creative with your basing/modeling you can take the scouts that are supposed to sit inside the transport, and put them on bases to add another 5 bodies to your army!

haha Just sit them on some rocks or crates or something). Don't forget that for 55 points though, 5 of them can outflank giving you line breaker and steal an objective fairly easily.
Devastators - Unfortunately I think you only get 2 lascannons in each box... but since you have 2 boxes you'll have 4. I would take lascannons all the time; 4 tank-hunter lascannon shots are great... almost a guaranteed kill on something. Save the rest of the bits honestly until you figure out how you like to run your Tactical Squads, and use them to fill in for a heavy weapon there.
Rhino - As for the rhino, just make sure to leave the top magnetized or just not glued down so that you can swap it with the razorback top at will (allows you to make both models in that kit). Also, I'd attach the smoke launchers and searchlight to it so that you don't forget it has them for free
lol! Also, build the little gunner with the storm bolter to put on top (don't glue the hatch on) so that if you run it as a Rhino you'll remember that it has that too for free (but not the razorback).
Tactical Squad - So this is where play-testing is going to determine what you build... leave a few without their special weapon (proxy them) until you find what you prefer. The main thing to think about is dedicating the squad to accomplishing a set task... pairing a special weapon and it's respective combi together, not including a heavy weapon on a unit in a rhino that has to be constantly moving, etc. Here are some load-outs I like:
* Plasma, Combi-Plasma, Rhino (10 marines) - good for always moving and getting into a position to shoot armor 2+ models or
MCs. Keep them back far enough from assault units, but remember they can move and rapid fire both plasma guns from the hatch. Don't bog them down with a heavy weapon. Melta bombs aren't necessary seeing as your objective is to keep them mobile and out of combat. Ringing in at a solid 200 points, it's my favorite option.
* Melta, Combi-Melta, Melta bombs, Rhino (10 marines) - Drive up close to another transport, prepare to get wrecked and then charged by the contents of the vehicle you explode (and possibly a
MC).
* Lascannon (5 marines) - For just 90 points, this unit sits back in cover (scores) on an objective and just annoys the enemy. Not a bad way to spend some leftover points. I wouldn't do this with any other special weapon though. The reason it works is the lascannon is just deadly enough to be a threat; other special weapons will be ignored.
* Avoid grav guns on Tactical Squads... they look great on paper, but you're probably going to always be moving... so you'll only get 2 shots and the range is worse than plasma.
* It's probably safe to assemble some marines with melta and combi-melta weapons, as you can always pick up a drop pod and use them as Sternguard (a great "alpha strike" unit is 5 in a drop pod with melta/combi-meltas).
HQ - This is the tough one... I'd say since you're just starting, use this guy as-is and give him some artificer armor. Until you define the real style of the army you're going for, picking an
HQ can be a challenge. Also, if this guy is a Captain, he unlocks the command squad... which is a great way to get 5 special weapons in one unit!
Hope that helps!
On a side note, if you haven't opened everything, maybe consider building the things above and return some unused things to a local
GW store for store credit and pick up some different things?
Automatically Appended Next Post: A decent starting list would be something like this:
HQ (140 points)
Captain - artificer armor, storm bolter, power fist
Troops (555 points)
Tactical Squad (10) - combi-plasma, plasma, rhino
Tactical Squad (10) - combi-plasma, plasma, rhino
Scout Squad (10) - all bolters, Land Speeder Storm w/heavy flamer (combat squad 5 in the speeder, and 5 outflanking)
Fast (125)
Stormtalon - Skyhammer missiles
Heavy (178 ponts)
Devastator Squad (7) - 4x lascannons, 3x bolters (just there for ablative wounds)
Total: 998 points
You can almost build this from what you have now, if you return a few things (again, not sure if it's an option). You would need to pick up a Stormtalon, Rhino, and Land Speeder Storm. If you returned the Combat Squad box for the Land Speeder Storm and 1 Tactical Squad for the Stormtalon, you would really only be looking at buying another Rhino. Just a thought.