At any other point in time, I would have told you to save rift coins for good catalysts first. Right now, considering we are about to have a revamp in the rift system, this may not be the best advice. Even so, depending on your level and access to rift coins, having enough good "all around" catas for at least your boss and inertia nodes may still be a good idea. Things like Blockbusted, Hexy, Frost armour and SlowMo that are 400 rift coins. But as Disaster told you. DONT BUY CATA RELICS. Always useless.
As mentioned by Tsukiakari, Yujipooji's rift series is a great read when you are starting. The main advice that I would take from there is:
- Start as early as possible, which you are doing now. This serves two purposes. First, you start getting the rewards and experience earlier. Second, it's actually easier if you start at a low level. As you'll be matched with people of similar roster. It's "normal" to see level 40/45 players at silver rankings and level 70 struggling to pass bronze due to that. Did I mention the rewards? XD
Besides that, look at what you are usually more annoyed by and try to look for specific counters against those things. Three "easy to get" fighters that you should consider to cover "most" scenarios are Sketchy, PuurD and Toad. Also, Primed is a must once you can get 10% in her PF.
If you have specific questions, don't hesitate to ask. And remember to enjoy it. This mode is the more frustrating, that's true. But it's also the only one that gives you a purpose to have a complete trained roster.