She's not very good in this game, but I play as her really often - here are some tips.

Your bread and butter combo is downthrow into an air. Up air fantastic for racking up damage and you can sort of juggle people around with it. Forward air is also good. After a downthrow --> f-air, if your opponent jumps, you can land another fair with a double jump. Dash attack can also lead into a forward air or upair.

Your charge shot is one of your most reliable kill moves, which is a little unfortunate considering how predictable it can be. The key is to use it to either apply mental pressure on your enemy and bait reflectors, dodges, shields, etc. and to fire it at unexpected times, or after a missed tech or something like that. One trick while charging is that you can cancel the charge with either a grab or a shield. I find the shield to be pretty versatile since you can cancel it into a jump immediately after - this gives you a lot of mobility and you can sometimes punish people who might try to approach you while charging. Alternatively, you can cancel your charge with a shield and then cancel the shield into an up special, but that's less useful. Also, try using shots that aren't fully charged sometimes to catch people off guard.

Your bombs are good for recovery and overall stage control. If you're knocked into the air, use them to protect your landing and to make yourself a smaller target.

At higher percents, your up special can kill - as ZeRo has mentioned, it's a solid out of shield option.

Samus's missiles kind of suck in this game but they still have applications. Use the slower, homing ones to apply pressure and stop approaches/mess with recoveries. The strong missiles are great for damaging shields and do good damage, so you might be able to get a shield break if you shoot a charge shot right after.

Samus's air game isn't too good but there are a few things you should note. A shorthop downair is a good option sometimes and even if you hit from the side, at higher percents, this can kill. A fast falled upair pretty much autocancels as you hit the ground so this can be an interesting mixup. Your nair can also kill at times, but I'm not experienced enough to do this consistently - I think the second hitbox has more knockback. Back air is also solid, but there is a lot of lag when you fast fall it. You can sometimes land a back air after a shorthopped d-air when the enemy bounces up.

Samus has some cool tilts - the downtilt comes out fast and pops people up, possibly leading to aerial followups. Forward tilt is a great spacing move - try using it when running backwards. Uptilt is weird but if you hit it, you can sometimes land another one. Samus's jab sucks because the second hit can be shielded and you'll get punished.

Last tool you should know about is the z-air. I don't use it that much for spacing, but I find it's a great tool when you are on the ledge. Hop down, then jump up and use the z-air to hit your enemy.